feat(harnessx): Phase 3 task 5.2 — SHA-pinned optional extra + capability guard

Wire HarnessX as an optional, SHA-pinned extra with a lazy capability guard,
keeping the core install zero-dependency.

- pyproject: add [project.optional-dependencies] harnessx pinned to commit
  274c512 (Darwin-Agent/HarnessX). Core dependencies stays []. uv_build emits
  `Requires-Dist: harnessx @ git+...@<sha> ; extra == 'harnessx'` +
  `Provides-Extra: harnessx` (verified via uv build); release ships to GitHub
  Releases, not PyPI, so the direct-reference is fine.
- harness_ext.py: the capability seam. has_harnessx() probes via find_spec
  (never imports harnessx); require_harnessx() imports lazily on demand and
  raises HarnessXUnavailableError with install guidance when absent. Every
  Phase 3+ HarnessX path routes through here so heavy imports stay cold by default.
- uv.lock refreshed: the universal lock now resolves the harnessx tree (149 pkgs)
  but `uv sync --group dev` installs only 57 — harnessx is NOT importable in the
  default/CI venv (verified), so CI stays lean and never re-locks.
- Tests: core deps empty; the extra is a 40-hex SHA-pinned git URL; importing
  openultrasast.cli does not import harnessx; has_harnessx probes without
  importing; require_harnessx raises with guidance when absent.

98 tests pass; ruff + mypy clean; detection gate 93.6%/0% intact. Spec task 5
(5.1 + 5.2) checked off; next is task 6 (build_sast hunter sub-harness).

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

.kiro/specs/harnessx-self-improving-rulesets/spec.json

@@ -1,7 +1,7 @@
 {
   "feature_name": "harnessx-self-improving-rulesets",
   "created_at": "2026-06-22T10:07:18Z",
-  "updated_at": "2026-06-22T14:05:00Z",
+  "updated_at": "2026-06-22T14:25:00Z",
   "language": "en",
   "phase": "implementation",
   "approvals": {