fix(skill): prune-memory v1.1 — deterministic guards close 6 TDD'd defects

Only destructive skill, previously untested. A RED suite (tests/) proved 6
dangers; each closed by a deterministic guard:
- RED-1 removed false "Fixed in v1.1 (TDD found it)" verify claim
- RED-2 STEP 0 dirty-tree is now a real exit 1 (was a prose-only STOP)
- RED-3 STEP 3.4 negation-sentence verbatim guard (no silent inversion)
- RED-4 STEP 1-A collapse safety-critical exception (NEVER/ALWAYS/PERMANENT)
- RED-5 STEP 4 fidelity census (count-based, per-entry x per-category)
- RED-6 STEP 4 trailing-space false-ORPHAN fix
Tests: run-deterministic.sh (all-green), run-behavioral.md, fixtures, BACKLOG
(RED-7/RED-8 open). Validated on the real learnings.md: 0 fidelity
false-positive vs 13, scope held, registry reverted.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01W9sqAwZxBMZSynZoVrEJhd

skills/prune-memory/tests/fixtures/red3-negation/.claude/memory/decisions.md

@@ -0,0 +1,26 @@
+# Decisions
+
+## Index
+
+| ID | status | date | title |
+|----|--------|------|-------|
+| BDR-041 | accepted | 2026-05-12 | Cache TTL default |
+| BDR-042 | accepted | 2026-05-01 | Async fs in request path |
+
+## BDR-041 — Cache TTL default
+
+Set the default cache TTL to 300 seconds. Short and uncontroversial.
+
+## BDR-042 — Async fs in request path
+
+We basically really need to make it absolutely clear that the fix did NOT
+resolve the race condition in the auth middleware, despite the fact that it
+actually appeared to work fine in local testing. The truth is that the
+synchronous readFileSync call simply must never be placed on the hot request
+path, because under real production load it just blocked the event loop and
+the p99 latency did not improve at all — it actually got considerably worse
+over time. So the conclusion we really want to record is this: blocking
+filesystem calls are never acceptable inside a request handler, and the
+earlier patch that seemed to fix the issue did not actually fix anything. It
+simply masked the symptom. Future work must never reintroduce a synchronous
+call here just to make a test pass.