enhance(#1964): semantic knowledge-base recall via MemPalace (keyword fallback) (#2416)

* test(#1964): add failing-first semantic-recall contract tests

Epic #1957 Phase 3C (final). Source-text-is-the-product contract tests:
semantic recall via MemPalace (top-k meaning-similar prior resolutions, catches
same-root-cause/different-wording cases), indexing resolved sessions at archive,
graceful degradation to keyword matching when MemPalace is absent,
knowledge-base.md stays the durable plain-text source of truth, agent Phase 0 /
Matching Logic is semantic-first (the stale 'keyword overlap, not semantic
similarity' claim must go), and no new embedding/vector infra (reuse MemPalace).

Failing-first: reference, the Matching Logic reframe, the Phase 0 consolidation,
and the archive indexing step do not yet exist.

* feat(#1964): semantic knowledge-base recall via MemPalace (keyword fallback)

Epic #1957 Phase 3C (FINAL). Replaces keyword-overlap matching with semantic
recall: at Phase 0 the debugger queries MemPalace with the current symptoms
and surfaces the top-k meaning-similar prior resolutions, catching the
same-root-cause/different-wording cases keyword overlap missed (the self-noted
'keyword overlap, not semantic similarity' limitation). Resolved sessions are
indexed into MemPalace at archive (symptoms + root_cause(s) + fix + recurrence
guard). knowledge-base.md remains the durable plain-text source of truth; when
MemPalace is absent the debugger falls back to keyword-overlap matching
(logged, never a silent skip). No new embedding/vector infrastructure —
MemPalace is reused.

Size-neutral agent edits: the Matching Logic section reframed (keyword-only ->
semantic-first + keyword-fallback + @-include); Phase 0's three keyword bullets
consolidated into one semantic-first bullet; one MemPalace-indexing step added
at archive. Agent at 57222 B (122 B headroom — final phase). Full rules in
gsd-core/references/debugger-semantic-recall.md. INVENTORY + manifest +
agent-size baseline + install-parity goldens + AGENTS.md updated.

* fix(#1964): address orthogonal review (invocation mechanism, index Resolution-not-symptoms + redaction, fallback detail)

- HIGH: the 'query MemPalace' instruction was WHAT-level only; the agent has
  no MCP tools. Added an Invocation section naming the Bash CLI
  (mempalace search --wing <wing>) + MCP-when-registered + wing resolution
  (config.mempalace.wing -> project_code -> project dir), matching every other
  MemPalace integration. Without this the feature silently degraded to keyword
  matching even when MemPalace was present.
- MEDIUM (security x2): index the agent-authored Resolution summary
  (root_cause + fix + recurrence_guard), NOT raw user-supplied Symptoms —
  excludes attacker-controlled prose from the cross-session index AND reduces
  secret/PII leakage. Redact secret-shaped values before indexing. Stated the
  write order (KB append + commit MUST succeed before indexing).
- LOW: restored 'identifiers' + 'case-insensitive' to the keyword fallback;
  added a test asserting the fallback mechanics survived the Phase 0
  consolidation (Error patterns field, 2+ token overlap, identifiers,
  case-insensitive).

* chore(#1964): ratchet agent-size baseline downward (leaner archive bullet shrank gsd-debugger.md 57222->57197)

* chore(#1964): backfill changeset pr number (PR #2416)

agents/gsd-debugger.md

@@ -822,9 +822,11 @@ At the **end of `archive_session`**, after the session file is moved to `resolve
 
 ## Matching Logic
 
-Matching is keyword overlap, not semantic similarity. Extract nouns and error substrings from `Symptoms.errors` and `Symptoms.actual`. Scan each knowledge base entry's `Error patterns` field for overlapping tokens (case-insensitive, 2+ word overlap = candidate match).
+**Semantic-first, keyword-fallback.** Query MemPalace with the current symptoms and surface the top-k meaning-similar prior resolutions — this catches same-root-cause/different-wording cases keyword overlap misses. Fall back to keyword overlap on `knowledge-base.md` when MemPalace is absent. See:
 
-**Important:** A match is a **hypothesis candidate**, not a confirmed diagnosis. Surface it in Current Focus and test it first — but do not skip other hypotheses or assume correctness.
+@~/.claude/gsd-core/references/debugger-semantic-recall.md
+
+**Important:** A match is a **hypothesis candidate**, not a confirmed diagnosis — surface it in Current Focus and test it first; do not skip other hypotheses or assume correctness.
 
 </knowledge_base_protocol>
 
@@ -984,9 +986,7 @@ At investigation decision points, apply structured reasoning:
 **Autonomous investigation. Update file continuously.**
 
 **Phase 0: Check knowledge base**
-- If `.planning/debug/knowledge-base.md` exists, read it
-- Extract keywords from `Symptoms.errors` and `Symptoms.actual` (nouns, error substrings, identifiers)
-- Scan knowledge base entries for 2+ keyword overlap (case-insensitive)
+- Query MemPalace semantically with the current symptoms (top-k meaning-similar prior resolutions); fall back to reading `.planning/debug/knowledge-base.md` and keyword overlap when MemPalace is absent
 - If match found:
   - Note in Current Focus: `known_pattern_candidate: "{matched slug} — {description}"`
   - Add to Evidence: `found: Knowledge base match on [{keywords}] → Root cause was: {root_cause}. Fix was: {fix}. Why not caught: {why_not_caught}. Recurrence guard: {recurrence_guard}.` (the last two are absent on old entries — that's fine; consume them when present)
@@ -1251,6 +1251,8 @@ Commit the knowledge base update alongside the resolved session:
 gsd_run query commit "docs: update debug knowledge base with {slug}" --files .planning/debug/knowledge-base.md
 ```
 
+**Index into MemPalace (when available)** per the semantic-recall reference — the Resolution summary (not raw symptoms), redacted — so a future Phase-0 query surfaces it by meaning. Skip with a logged note when MemPalace is absent or the KB write failed; `knowledge-base.md` is the durable fallback.
+
 Report completion and offer next steps.
 </step>