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

tests/conftest.py

@@ -5,12 +5,6 @@ import sys
 
 import pytest
 
-_HARNESSX_GUARD = (
-    "\nimport sys as _sys\n"
-    "_leaked = sorted(m for m in _sys.modules if m == 'harnessx' or m.startswith('harnessx.'))\n"
-    "assert not _leaked, 'leaked harnessx modules: ' + repr(_leaked)\n"
-)
-
 
 @pytest.fixture(autouse=True)
 def _pointer_pairs_offline(monkeypatch: pytest.MonkeyPatch, tmp_path_factory: pytest.TempPathFactory) -> None:
@@ -31,17 +25,16 @@ def _repos_offline(monkeypatch: pytest.MonkeyPatch, tmp_path_factory: pytest.Tem
 
 
 @pytest.fixture
-def assert_cold_of_harnessx():  # type: ignore[no-untyped-def]
-    """Run ``code`` in a fresh interpreter and assert it imported no ``harnessx`` module.
+def assert_cold_import():  # type: ignore[no-untyped-def]
+    """Run ``code`` in a fresh interpreter and assert it exits 0.
 
-    Hermetic by construction: the lazy-import guarantee is verified the same way
-    whether or not the optional extra is installed in this environment (a global
-    ``sys.modules`` check would be polluted by other tests once the extra is present).
+    Hermetic by construction: an import-laziness check (``code`` asserts on ``sys.modules`` itself) is not
+    polluted by modules other tests have already imported into this process.
     """
 
     def _check(code: str) -> str:
         proc = subprocess.run(
-            [sys.executable, "-c", code + _HARNESSX_GUARD],
+            [sys.executable, "-c", code],
             capture_output=True,
             text=True,
         )