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).

tests/fixtures/install-tree/augment.json

@@ -131,6 +131,7 @@
   "gsd-core/references/common-bug-patterns.md",
   "gsd-core/references/context-budget.md",
   "gsd-core/references/continuation-format.md",
+  "gsd-core/references/debugger-fix-acceptance.md",
   "gsd-core/references/debugger-philosophy.md",
   "gsd-core/references/decimal-phase-calculation.md",
   "gsd-core/references/doc-conflict-engine.md",