feat(#1958): add multi-signal fix-acceptance guardrail to gsd-debugger (#2396)

* test(#1958): add failing-first guardrail contract tests

Epic #1957 Phase 1A. Adds source-text-is-the-product tests asserting the
5-signal fix-acceptance guardrail contract (target test, mutation check,
no-op/deletion detector, adjacent tests, revert-and-reconfirm), graceful
degradation, FIX REJECTED BY GUARDRAIL return path, per-signal debug-file
recording, and subprocess bounding.

Failing-first: reference file and agent sections do not yet exist.

* feat(#1958): add multi-signal fix-acceptance guardrail to gsd-debugger

Epic #1957 Phase 1A. Prevents accepting a fix that merely greens the test
(Goodhart defense / APR overfitting). Adds a 5-signal gate run before fix
acceptance: target test, mutation check (Stryker), no-op/behavior-deleting
detector, adjacent/held-out tests, revert-and-reconfirm. Degrades gracefully
when Stryker or a test suite is absent (each skip logged, never a silent pass),
records per-signal results under Resolution.verification, and returns a
FIX REJECTED BY GUARDRAIL outcome the session-manager surfaces for
revise / accept-as-debt / abandon.

Full rules extracted to gsd-core/references/debugger-fix-acceptance.md (slim
routing kept in the agent to respect the agent-size cap). Debug template +
INVENTORY + manifest + agent-size baseline + AGENTS.md updated.

* test(#1958): correct newline-tolerant assertion + regen install-parity goldens

The revert-and-reconfirm assertion collapsed whitespace before matching so
markdown line-wrapping does not break it. Regenerated the golden-install-parity
and install-tree fixtures (npm run gen:golden) to absorb the intentional
gsd-debugger.md / gsd-debug-session-manager.md / DEBUG.md / new reference-file
changes to the installed artifact tree.

* fix(#1958): tighten guardrail per orthogonal review

Addresses the isolated reviewer's findings:
- signal 5 now states its recorded-repro dependency and routes the no-repro
  case to the degradation row; revert mechanism specified (git stash / git
  revert -n); minimality flag tied to diff structure, not revert-ability.
- bounded-subprocesses section now bounds the git subprocess (5-30s) too,
  requires argv-array argument passing, and scopes Stryker to the driving
  regression test (a mutant killed only by a non-driving test is a finding).
- new test-provenance (security) clause: the driving test must be
  agent-authored; bug-report repro scripts are DATA, never executed verbatim.
- tightened 3 contract assertions to bind to specific clauses
  (guardrail_verdict field, deletion-reject-unless-RCA, 60s+git bounding).
- Goodhart framing softened to 'partially-independent'; DEBUG.md template
  verification field notes the nested map shape.

* chore(#1958): backfill changeset pr number (PR #2396)

* fix(#1958): add issue ref to allow-test-rule annotation (ADR-456)

CI lint-allow-test-rule-refs requires every allow-test-rule exemption to
carry a 'see #NNN' issue ref per ADR-456. The new test file's annotation
lacked it; this adds (see #1958).

agents/gsd-debugger.md

@@ -1091,11 +1091,15 @@ Update status to "fixing".
 - Make SMALLEST change that addresses root cause
 - Update Resolution.fix and Resolution.files_changed
 
-**2. Verify**
+**2. Verify (Fix-Acceptance Guardrail)**
 - Update status to "verifying"
-- Test against original Symptoms
-- If verification FAILS: status -> "investigating", return to investigation_loop
-- If verification PASSES: Update Resolution.verification, proceed to request_human_verification
+- Run the multi-signal guardrail before accepting the fix:
+
+@~/.claude/gsd-core/references/debugger-fix-acceptance.md
+
+- Record every signal's result under `Resolution.verification` (per-signal schema in the reference)
+- If ANY applicable signal fails (and no documented technical-debt escape applies): return `## FIX REJECTED BY GUARDRAIL` (see structured_returns) — do NOT request human verification
+- If all applicable signals pass: set `guardrail_verdict: accepted`, proceed to request_human_verification
 </step>
 
 <step name="request_human_verification">
@@ -1334,6 +1338,16 @@ Orchestrator presents checkpoint to user, gets response, spawns fresh continuati
 
 Only return this after human verification confirms the fix.
 
+## FIX REJECTED BY GUARDRAIL
+
+Returned when a fix-acceptance guardrail signal fails (see `@~/.claude/gsd-core/references/debugger-fix-acceptance.md`). Do **not** mark the session resolved.
+
+**Debug Session:** .planning/debug/{slug}.md
+**Failing signal:** {signal 1–5 name}
+**Evidence:** {why the signal failed — e.g. "mutant at fix site survived", "deletion-only diff with no RCA justification", "bug did not return on revert"}
+
+The session-manager continuation surfaces this and offers revise / accept-as-debt / abandon.
+
 ## INVESTIGATION INCONCLUSIVE
 
 ```markdown