feat(skills): Phase 14 — Trail of Bits skill index + router

Replace the hardcoded skill-name stub with a real, routable skill index.

- skills.py (zero-dep): SkillDescriptor inventory (~22 ToB skills with routing
  metadata — languages, tags, vulnerability classes, stages — plus a short operating
  checklist each), aligned with the spec's routing examples. route_skills scores by
  vulnerability-class/tag/stage signal with a language filter+bonus, and never selects
  on a pure language match. route_skills_for_target derives vuln classes from tags;
  select_skill_context packs checklists into a char budget; skill_chunks chunks the
  guidance into the `skills` retrieval namespace.
- hunter.select_skill_snippets now delegates to the router (existing C-memory and
  crypto+auth routing expectations preserved).
- hunter_harness._describe feeds the budget-capped routed skill context into the
  HarnessX hunter prompt (the genuine consumer of skill_budget_chars).
- 8.4 zero-dep guard extended to cover skills.py.

tests/test_skills.py (9 tests: C parser → memory/fuzzing, crypto → constant-time/
vector, language-mismatch + language-only exclusion, stage filtering, budget cap,
namespace chunking). openrouter-sast-harness Phase 14 checked off.

188 passed, 4 skipped; ruff + mypy clean; detection gate PASS.

Co-Authored-By: Claude Opus 4.8 <[email protected]>

tests/test_gate.py

@@ -147,6 +147,7 @@ GOVERNANCE_SCORING_BENCHMARK_PLANES = (
     "openultrasast.slot_contract",
     "openultrasast.stage_processors",
     "openultrasast.fusion",
+    "openultrasast.skills",
 )