Add getGitDiff read action and fix code-agent error semantics

[George Ng (GeorgeNgMsft)]

What this does

Two small, related fixes to the Code Agent / Coda (VS Code extension bridge), both read-only:

1. New getGitDiff action — a deterministic, catalog-discoverable fast action for the code agent that returns the actual local Git diff: changed files plus bounded unified-diff patch/hunk text, split into staged and unstaged sections by default, or diffed against an explicit base ref (base parameter) when given. It selects a repository explicitly or falls back to the single open repo/workspace folder (repository parameter, for multi-root workspaces). Binary files, renames, copies, and deletions are reported with clear status/flags instead of garbled or dropped patch text, and output is bounded (per-file byte cap, per-section byte cap, file-count cap) with truncation flags so callers always get a well-formed, complete response instead of a runaway payload.
2. Fixed error semantics — when a Coda read action fails for an operational reason (e.g. no repo open, an unhandled action), the Code Agent now returns a failed ActionResult instead of a success-shaped result that merely happens to contain {"error": ...} in its text.

getWorkspaceChanges is unchanged.

Why this matters for external MCP clients

getGitDiff needs no separate MCP wiring — it's defined the same way as every other code-agent action, so it's automatically visible through the existing active schema/catalog (the same pipeline packages/actionSchema/test/regen.spec.ts round-trips and commandExecutor reads for discovery). A new test (codeActionsSchemaDiscovery.spec.ts) proves it's present with a concise description and a stable, typed parameter list — deterministic and usable by an external tool-selecting client without going through TypeAgent's natural-language reasoning path.

Highlights

• packages/coda/src/gitDiffUtils.ts (new) — pure, unit-tested diff parsing: quoted/unquoted Git header paths (including non-ASCII under both core.quotePath settings), trailing-tab disambiguation on spaced paths, binary/mode-only/rename/copy/deletion handling, and bounded truncation with reported caps.
• packages/coda/src/handleReadActions.ts — getGitDiff uses a single repo.diffWith() call per section (no serial per-file calls), filters output to paths inside an open workspace folder (checking both current and old paths for renames/copies), and reuses a shared getGitApi() helper.
• packages/agents/code/src/codeActionsSchema.ts — new GetGitDiffAction schema with a concise, standalone description and base/repository parameters.
• packages/agents/code/src/codeActionHandler.ts — operational failures (JSON {error} results, and the "action not handled" fallback) now produce a failed ActionResult.
• Tests: 28 new unit tests in packages/coda/test/gitDiffUtils.spec.ts (new test:local harness for coda, matching existing agent conventions) and 2 new suites in packages/agents/code/test/ (codeActionErrorSemantics.spec.ts, codeActionsSchemaDiscovery.spec.ts) — 31/31 code-agent tests, 28/28 coda tests passing.
• All CLAUDE.md ratchet gates (lint, complexity, circular deps, tech debt) pass against main.

Co-authored-by: Copilot App 223556219+Copilot@users.noreply.github.com

[George Ng (GeorgeNgMsft)]

Closing for now, will reopen when ready for review