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

plane/Dockerfile.runner

@@ -20,7 +20,7 @@ RUN apt-get update \
 WORKDIR /opt/openultrasast
 COPY pyproject.toml README.md ./
 COPY src ./src
-# Core install only: PyYAML for the manifests, no extras (no HarnessX, no tree-sitter).
+# Core install only: PyYAML for the manifests, no extras (no tree-sitter).
 RUN pip install --no-cache-dir . \
  && printf '#!/bin/sh\nexec python -m openultrasast.plane.runner "$@"\n' > /usr/local/bin/ax-task-runner \
  && chmod 0755 /usr/local/bin/ax-task-runner