chore(harnessx-removal): delete HarnessX code and packaging (task 6)

Deletes harness_ext.py and hunter_harness.py with their HarnessX-only
tests (the judge path, verify_judge.py, went in task 5). The harnessx
extra, its mypy override and its uv.lock entries are removed (uv lock
drops only packages; no remaining version changes). The conftest
fixture becomes assert_cold_import; test_gate's two never-import checks
become one allow-list (stdlib, openultrasast, yaml) over the governance
planes' module-level imports, and the skipped live-extra tolerance test
is deleted. Audit rows for the two modules are removed, stage_processors
is reworded, and the module-audit manifest is regenerated (no orphan).

test_removed_plane_references implements Req 2.4 over src, tests and
pyproject.toml: a match must carry a `retired 2026-..` tag (the spec's
own slug is not counted). The README part is a strict xfail until task 8
rewrites the README's plane sections.

Co-Authored-By: Claude Opus 5.5 <[email protected]>
Claude-Session: https://claude.ai/code/session_01EmM1j46qgKMh3zqD1QMwEo

.kiro/specs/harnessx-removal/tasks.md

@@ -44,7 +44,7 @@ and passed its fixture test. Every task is a commit gated on the full suite's ow
     tagged retirement text allowed in `src`, enforced by a test); tests renamed to the behaviour they assert.
   - _Requirements: 1.1, 1.2, 1.3, 3.3_
 
-- [ ] 6. Delete HarnessX code and packaging
+- [x] 6. Delete HarnessX code and packaging
   - Delete `harness_ext.py`, `hunter_harness.py`, `verify_judge.py`'s judge path, the HarnessX-only tests; remove
     the extra, the mypy override and the `uv.lock` entry; rename the conftest fixture; audit rows and manifest; the
     reference search test (Req 2.4).