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

src/openultrasast/gate.py

@@ -171,7 +171,7 @@ def within_tolerance(
 ) -> bool:
     """True if ``candidate`` recall/FP stay within tolerance of ``baseline``.
 
-    Used to confirm a HarnessX-backed stage does not drift detection beyond the
+    Used to confirm a model-backed stage does not drift detection beyond the
     configured band of the zero-dependency baseline. Recall may not fall more than
     ``recall_tolerance`` below baseline; FP may not rise more than ``fp_tolerance``
     above it.