test: delete test-infrastructure dead code (G)

- exit-propagation drives the runner's real strict verdict
  (BunTestOutputClassifier + strictTestExitCode); delete the unused
  shardRunLooksTruncated predicate.
- delete skill-coverage-matrix registry + its gate (nothing reads it; the
  floor already iterates skillCensus()).
- delete touchfiles-facade export-parity tests (Bun fails missing imports
  at link time) and the duplicated E2E_TIERS tier-value test.
- delete brain-cache-spec TRANSPORT_DEFAULT_POLICY, SKILL_RUN_RETENTION_DAYS
  and the now-unused BrainTrustPolicy type with their literal tests.
  AUTOPLAN_PREFLIGHT_BUDGET_BYTES stays: skill-preflight-budget enforces it
  against real resolver output.
- delete audit-compliance's JSDoc-comment grep.

test/gstack-schema-pack.test.ts

@@ -4,7 +4,7 @@
  * Asserts the schema pack is well-formed and matches the v1.48 plan:
  *   - Exactly 8 page types (7 entities + 1 take)
  *   - Frontmatter shape is internally consistent
- *   - Retention policies match SKILL_RUN_RETENTION_DAYS spec
+ *   - Retention policies (skill-run pages archive after 90 days)
  *   - Link verbs only reference declared verbs
  *   - JSON payload shape is acceptable to mcp__gbrain__schema_apply_mutations
  *