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/model/audit.py

@@ -77,9 +77,7 @@ STANDALONE = {
     "mcp": "narrow MCP server over stdio; its own `ousast mcp` entry point and OpenCode integration",
     "skills": "skill router; consumed by mcp and by the OpenCode integration, not by the scan path",
     "fusion": "two-panel adjudication engine; reached from the scan's report stage and used standalone",
-    "harness_ext": "HarnessX extension surface; loaded by name when the optional harness is present",
-    "hunter_harness": "harness wiring for the tool hunter; used by the hunter path and by mcp",
-    "stage_processors": "HarnessX slot-contracted scan stages; pinned by the zero-dependency guard in test_gate",
+    "stage_processors": "deterministic slot-contracted scan stages; pinned by the zero-dependency guard in test_gate",
     "slot_contract": "the slot/contract protocol stage_processors implements",
 }