feat(agentfabric): schema-driven graph extraction + graph-aware lint rules

[Drake-Ash]

What

Expands the AgentFabric dialect with new graph-aware lint rules, a schema-driven
graph-extraction library, and the @agentscript/language primitives those features
depend on.

The diff spans three layers:

1. AgentFabric lint — five new rules (cycle detection, execute-body validation,
   action-binding validation, unused-node, strict schema validation) and removal
   of two deprecated router checks.
2. New graph library (dialect/agentfabric/src/graph/) — extracts a directed
   node/edge graph from a parsed document purely from schema metadata, plus an
   adapter to the agent-graph canvas protocol.
3. @agentscript/language core — new schema field markers, a unified completions
   resolver, and cycle-safe schema walkers.

How

New lint rules (src/lint/passes/rules/)

• cycle-rules.ts — Detects execution-graph cycles via iterative DFS over extracted
  edges, deduped by a rotation-invariant signature. Warns on each cycle member
  (cycle-detected), e.g. Cycle detected in execution flow: @a → @b → @a.
  Triggers are excluded from membership but seed the traversal.
• execute-rules.ts — Validates each executor do body: set targets must be
  @variables./@outputs.; run targets must reference an @actions.* def whose
  kind is a2a:send_message or mcp:tool; with params must match declared inputs;
  bare @ and @actions.* are rejected as values.
• action-binding-rules.ts — For orchestrator/subagent/generator nodes, validates
  every with <param> in a reasoning.actions binding is a declared input (or
  implicit param).
• unused-node.ts — Flags declared-but-unreferenced nodes as an Information
  diagnostic tagged Unnecessary, with a removalRange for quick removal.
• strict-schema-validation.ts — Promotes unknown-field/unknown-block from
  Warning → Error (AgentFabric is a strict compile target).
• Also enables the core unusedVariablePass for the dialect.
  Removed: deprecated switch-choices-deprecated and switch-on-exit router checks.

New graph library (src/graph/)

• extractor.ts — Schema-driven extractor. Nodes/edges are derived from the
  transitionTarget capability and transitionContainer/transitionTarget field
  markers — no block/field names hardcoded. Surfaces lexical ranges, sibling
  primitives, and predicates.
• get-graph.ts — Adapter to the agent-graph canvas protocol ({nodes, edges} with
  first-class kind + open additionalProperties). Documents a comma-escaping
  grammar for the joined outputs summary that must stay in sync with the UI parser.
• extractGraph/getGraph re-exported from the package entry point.

@agentscript/language core

• New FieldMetadata flags + FieldBuilder methods: transitionContainer(),
  predicateField(), outputNameField(), displayLabelField().
• Completions refactor — getFieldCompletions and getValueCompletions now share one
  walkParentsToSchemaContext resolver (the two prior divergent walkers were a bug
  source). Value completions are discriminant-variant aware and add quoted
  enum-member completions via insertText. getValueCompletions now takes ast.
• scope.ts walkers gain a WeakSet visited-guard for self-referential schemas.
• Exports isAstNodeLike and the Range type.

Schema changes

• AgentFabric: OutputPropertyBlock is now genuinely self-referential (recursive
  items/properties) via lazy proxy field wrappers; adds includeDefault. Applies
  the new field markers across node blocks. Adds XHIGH to reasoning_effort.
  Removes explicit http_headers (now an implicit with param).
• AgentScript: marks baseSubagentFields.label as displayLabelField so cloned
  orchestrator nodes surface their label in the extracted graph.

Tests

New suites cover the extractor, protocol adapter, LLM variant/value completions,
snippet indentation, and the new lint rules.

Test Plan

• Existing tests pass (pnpm test)
• New/updated tests cover the change
• Linting and type checks pass (pnpm lint && pnpm typecheck)

Checklist

• My code follows the project's coding style
• I have reviewed my own diff
• I have added/updated documentation as needed
• This change does not introduce new warnings