test(#2974): migrate 8 test files to typed-IR assertions (#3016)

* test(#2974): migrate 8 test files to typed-IR assertions

Replaces raw stdout/stderr substring matching with structured-field
assertions per CONTRIBUTING.md "Prohibited: Raw Text Matching on Test
Outputs". Adds shared infrastructure for typed error emission so this
pattern is the easy path going forward.

Shared infrastructure:
- core.cjs: ERROR_REASON frozen enum + setJsonErrorMode/getJsonErrorMode
- gsd-tools.cjs: --json-errors CLI flag, parsed before subcommand dispatch
- config.cjs: typed reasons at all 7 error sites
- graphify.cjs: GRAPHIFY_REASON enum + reason/timeout_ms in execGraphify result
- bin/install.js: pure buildSdkFailFastReport() IR builder + renderer
- hooks/gsd-session-state.sh, gsd-phase-boundary.sh: emit Claude Code
  hookSpecificOutput JSON envelope with typed state_present/config_mode/
  planning_modified/file_path fields (no-op when hooks.community is off)

Test migrations (all pass, 171 tests across the 8 files):
- bug-2649-sdk-fail-fast: assert on ir.reason / ir.context / ir.fix_command
- bug-2687-config-read-warning-parity: assert.equal stderr === ''
- bug-2796-arg-parsing-regression: assert on result.json.updated/.phase
- bug-2838-summary-rescue: parse rescue footer, assert mtime invariant
- bug-2943-config-get-context-window: parse JSON, assert ERROR_REASON.CONFIG_KEY_NOT_FOUND
- graphify: assert reason === GRAPHIFY_REASON.ENOENT/TIMEOUT
- hooks-opt-in: parse hookSpecificOutput, assert typed fields
- security-scan: reclassified as source-text-is-the-product (scan label
  output and CI workflow YAML ARE the deployed contract)

Verification: lint-no-source-grep clean (0 violations), full suite
6741/6741 pass.

Closes #2974

* test(#2974): address CR feedback — typed code field, robust idempotency

Two CodeRabbit findings on #3016 addressed:

1. tests/hooks-opt-in.test.cjs:355 (Minor, inline) —
   parsed.reason.includes('Conventional Commits') was still substring
   matching after the typed-IR migration. Fixed at the source: the
   gsd-validate-commit hook now emits a typed `code` field
   ('CONVENTIONAL_COMMITS_VIOLATION', 'COMMIT_SUBJECT_TOO_LONG')
   alongside the human-readable `reason`. Test asserts strictEqual
   on the code; the prose copy is no longer part of the test contract.

2. tests/bug-2838-summary-rescue-gitignored-planning.test.cjs:224-250
   (Outside-diff) — mtimeMs alone can stay unchanged on coarse-grained
   filesystems (HFS+, FAT) when two rewrites land within the same
   timestamp tick, falsely passing the idempotency assertion.
   Replaced with a full snapshot (mtimeMs, ctimeMs, size, ino, sha256
   of contents) compared via assert.deepStrictEqual — the hash
   catches any rewrite the timestamp would miss.

Verification: 30/30 pass on the two affected files; lint-no-source-grep
clean (0 violations across 368 test files).

get-shit-done/bin/gsd-tools.cjs

@@ -305,6 +305,18 @@ async function main() {
   const raw = rawIndex !== -1;
   if (rawIndex !== -1) args.splice(rawIndex, 1);
 
+  // --json-errors: when present, error() emits structured JSON to stderr
+  // ({ ok: false, reason: <ERROR_REASON code>, message }) instead of plain
+  // "Error: <text>". Lets test suites assert on typed reason codes per the
+  // CONTRIBUTING.md "Prohibited: Raw Text Matching on Test Outputs" rule
+  // (#2974). Default off — human operators see the original plain-text
+  // diagnostic.
+  const jsonErrorsIdx = args.indexOf('--json-errors');
+  if (jsonErrorsIdx !== -1) {
+    core.setJsonErrorMode(true);
+    args.splice(jsonErrorsIdx, 1);
+  }
+
   // --pick <name>: extract a single field from JSON output (replaces jq dependency).
   // Supports dot-notation (e.g., --pick workflow.research) and bracket notation
   // for arrays (e.g., --pick directories[-1]).