feat(core): add the file contract parser and evaluator

docs/file-contract.md

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+# File contract
+
+A file contract states, per path, what DeepCode may read, write, or execute. It
+is optional: with no contract file, nothing changes.
+
+It exists because `settings.json` permission rules match on the **tool**, not the
+path. `Bash(git diff:*)` and `WebFetch(domain:github.com)` work well, but there
+is no way to write "never read `.env`" — the only path-aware match is a prefix
+compare against the tool's primary argument, and a real `file_path` is usually
+absolute, so `Read(.env*)` matches nothing at all.
+
+> **This is policy, not a security boundary.** A contract constrains tool calls
+> that go through DeepCode's dispatcher. It does not constrain what a shell
+> command does after Bash starts — `cat .env` is a string, and statically
+> analysing shell to decide otherwise would be guesswork that reads as a
+> guarantee. Only the **sandbox** bounds Bash. See
+> [security-model.md](security-model.md).
+
+## Where it lives
+
+The first file found wins; there is no merging.
+
+1. `<project>/.deepcode/file-contract.yaml`
+2. `<project>/.deepcode/file-contract.yml`
+3. `~/.deepcode/file-contract.yaml`
+4. `~/.deepcode/file-contract.yml`
+
+Project beats user rather than merging, so "which file denied this?" is always
+answerable by opening one file.
+
+## Format
+
+```yaml
+version: 1
+
+defaults:
+  read: allow
+  write: allow
+  execute: allow
+
+rules:
+  - glob: '**/.env*'
+    owner: human
+    read: deny
+    write: deny
+    reason: 'Secrets are human-only.'
+
+  - glob: '{AGENTS.md,CLAUDE.md,DEEPCODE.md}'
+    owner: shared
+    write: ask
+    reason: 'Agent instructions shape every future run — review before writing.'
+```
+
+| Field                    | Values                         | Meaning                                                      |
+| ------------------------ | ------------------------------ | ------------------------------------------------------------ |
+| `glob`                   | pattern                        | Which paths this rule covers (workspace-relative)            |
+| `read` `write` `execute` | `allow` \| `ask` \| `deny`     | Decision for that axis; omit an axis to say nothing about it |
+| `owner`                  | `human` \| `agent` \| `shared` | Responsibility, not access control — it shapes wording       |
+| `reason`                 | free text                      | Shown verbatim when the rule produces `ask` or `deny`        |
+
+`ask` is the useful middle state: the change is legitimate but wants eyes on it
+before it lands. Without it, everything high-impact has to be either waved
+through or forbidden.
+
+### Glob syntax
+
+| Pattern  | Matches                                |
+| -------- | -------------------------------------- |
+| `*`      | Any characters within one path segment |
+| `**`     | Any characters across segments         |
+| `a/**/b` | Also matches `a/b` — zero directories  |
+| `?`      | Exactly one non-separator character    |
+| `{a,b}`  | Either alternative                     |
+
+Everything else is literal, including `.`, so `**/.env*` cannot accidentally
+match `axenv`.
+
+### Precedence
+
+1. The **more specific** glob wins — fewer `**`, then more path segments, then
+   more literal characters.
+2. On an exact tie, the **later** rule wins.
+
+So a broad rule can be narrowed further down the file without reordering.
+
+### Paths outside the workspace
+
+A contract has no authority over `/etc`, so paths resolving outside the project
+get no verdict at all and fall through to the tool rules and the sandbox.
+
+Note that path resolution is string math — it does not call `realpath`. A
+symlink inside the workspace pointing outside still looks inside. This is the
+same reason the box at the top matters: the sandbox is the boundary.
+
+## Self-protection
+
+Writes to `.deepcode/file-contract.yaml` (and `.yml`) are always denied,
+regardless of what the file says. A contract that can grant itself
+`write: allow` is not a contract. Reading it stays allowed — auditing it is the
+whole point.
+
+## When it is malformed
+
+An unparseable contract is reported as **invalid**, not treated as absent.
+Falling back to "no contract" would silently drop every `deny` the author wrote,
+which is the worst possible failure for this particular file. DeepCode keeps
+running under the tool rules alone and says so, naming the file and line.
+
+The parser is strict on purpose: unknown keys, unknown decision values, a rule
+with no `glob`, or a rule that decides nothing are all errors. A silently-ignored
+line here is a permission quietly granted.
+
+## Interaction with `settings.json`
+
+The two rule sets compose by **most-restrictive-wins**:
+
+```
+final = mostRestrictive(toolVerdict, pathVerdict)      deny > ask > allow
+```
+
+A contract can only tighten. It never overrides a `deny` in `settings.json` into
+an allow, and an absent contract yields no verdict at all — which is what makes
+"no contract file, no behaviour change" exactly true rather than approximately
+true.

packages/core/src/config/file-contract-loader.ts

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+// Loading side of the file contract — kept apart from `file-contract.ts` so the
+// decision logic stays free of `node:fs` and remains exhaustively testable.
+// Plan: docs/FLOATBOAT_ADOPTION_PLAN.md §2.A
+
+import { promises as fs } from 'node:fs';
+import { homedir } from 'node:os';
+import { join } from 'node:path';
+import {
+  FileContractError,
+  parseFileContract,
+  type ContractDecision,
+  type FileContract,
+} from './file-contract.js';
+
+/**
+ * Outcome of looking for a contract.
+ *
+ * `invalid` exists because the two obvious alternatives are both wrong: falling
+ * back to "no contract" silently drops every `deny` the author wrote, and
+ * refusing to start turns a typo into a broken install. Reporting it lets the
+ * caller keep working under the tool rules alone while saying so loudly.
+ */
+export type FileContractStatus = 'absent' | 'loaded' | 'invalid';
+
+export interface LoadedFileContract {
+  status: FileContractStatus;
+  contract?: FileContract;
+  /** Absolute path of the file used, when one was found. */
+  path?: string;
+  /** Parse failure detail, present only when status is `invalid`. */
+  error?: string;
+}
+
+export interface LoadFileContractOpts {
+  cwd: string;
+  /** Override $HOME (tests). */
+  home?: string;
+  /** Direct DeepCode data directory (contains file-contract.yaml). */
+  directory?: string;
+}
+
+/** Candidate locations, most specific first. */
+export function fileContractPaths(opts: LoadFileContractOpts): string[] {
+  const home = opts.home ?? homedir();
+  const directory = opts.directory ?? join(home, '.deepcode');
+  return [
+    join(opts.cwd, '.deepcode', 'file-contract.yaml'),
+    join(opts.cwd, '.deepcode', 'file-contract.yml'),
+    join(directory, 'file-contract.yaml'),
+    join(directory, 'file-contract.yml'),
+  ];
+}
+
+/**
+ * Load the first contract that exists.
+ *
+ * Project beats user rather than merging them. Merging two rule lists would
+ * make precedence depend on concatenation order across files nobody sees
+ * together, and "which file denied this?" is a question the user has to be able
+ * to answer by opening one file.
+ */
+export async function loadFileContract(opts: LoadFileContractOpts): Promise<LoadedFileContract> {
+  for (const path of fileContractPaths(opts)) {
+    let raw: string;
+    try {
+      raw = await fs.readFile(path, 'utf8');
+    } catch (err) {
+      if ((err as NodeJS.ErrnoException).code === 'ENOENT') continue;
+      return { status: 'invalid', path, error: (err as Error).message };
+    }
+    try {
+      return { status: 'loaded', path, contract: parseFileContract(raw) };
+    } catch (err) {
+      const message =
+        err instanceof FileContractError ? err.message : `unparseable contract: ${String(err)}`;
+      return { status: 'invalid', path, error: message };
+    }
+  }
+  return { status: 'absent' };
+}
+
+/**
+ * Starter contract for `deepcode contract init`.
+ *
+ * Defaults stay `allow` on all three axes. A coding agent that writes code is
+ * doing its job, so the useful contract denies the handful of paths that are
+ * never the job, rather than asking about everything and training the user to
+ * approve reflexively.
+ */
+export const RECOMMENDED_FILE_CONTRACT = `# DeepCode file contract — permission rules on the path axis.
+# Docs: https://github.com/oratis/deepcode/blob/main/docs/file-contract.md
+#
+# Decisions: allow | ask | deny. Axes: read | write | execute.
+# More specific glob wins; equal specificity means the later rule wins.
+#
+# This constrains tool calls (Read/Write/Edit/Grep/Glob). It does NOT constrain
+# what a shell command does once Bash starts — only the sandbox does that.
+
+version: 1
+
+defaults:
+  read: allow
+  write: allow
+  execute: allow
+
+rules:
+  # Secrets are never the job.
+  - glob: "**/.env*"
+    owner: human
+    read: deny
+    write: deny
+    reason: "Secrets are human-only."
+
+  - glob: "**/*.{pem,key,p12,pfx,keystore,jks}"
+    owner: human
+    read: deny
+    write: deny
+    reason: "Private keys are human-only."
+
+  - glob: "**/{id_rsa,id_ed25519,id_ecdsa,.npmrc,.pypirc,.netrc}"
+    owner: human
+    read: deny
+    write: deny
+    reason: "Credential file — human-only."
+
+  # High impact: allowed, but worth a look before it lands.
+  - glob: "{AGENTS.md,CLAUDE.md,DEEPCODE.md}"
+    owner: shared
+    write: ask
+    reason: "Agent instructions shape every future run — review before writing."
+
+  - glob: ".github/workflows/**"
+    owner: human
+    write: ask
+    reason: "CI runs with repository credentials."
+
+  - glob: ".deepcode/settings.json"
+    owner: human
+    write: ask
+    reason: "Settings hold the permission rules themselves."
+`;
+
+/** Decisions in this contract that only take effect while the sandbox is on. */
+export function contractNeedsSandbox(contract: FileContract | undefined): boolean {
+  if (!contract) return false;
+  const denies = (d: ContractDecision | undefined): boolean => d === 'deny';
+  return (
+    contract.defaults.read === 'deny' ||
+    contract.rules.some((r) => denies(r.read) || denies(r.execute))
+  );
+}