docs: purge stale xdist/_enforce_test_timeout test-runner references repo-wide

The test runner moved to per-file subprocess isolation via
scripts/run_tests_parallel.py (hermetic `env -i`, worker count auto-scaled
from CPU count, FLAKY-retry policy) — no pytest-xdist, no SIGALRM per-test
timeout fixture. Docs still described the old runner in many places:

- AGENTS.md: "-n auto xdist workers, in-tree subprocess-isolation plugin"
  clause replaced with the current per-file-subprocess description; the
  `::test_x` single-test example now shows file + -k (runner is
  file-granular).
- CONTRIBUTING.md: "hermetic env, 4 xdist workers" comment corrected;
  `tests/conftest.py::_enforce_test_timeout` reference redirected to the
  win32 timeout-method shim in `tests/conftest.py::pytest_configure`.
- skills/autonomous-ai-agents/hermes-agent/references/contributor-guide.md
  and windows-quirks.md: same corrections (the bundled skill mirrors the
  contributor docs); Windows workaround no longer installs pytest-xdist
  or passes -n 0.
- website/docs + zh-Hans i18n mirrors: same fixes in adding-providers.md
  and the bundled-skill doc pages.
- skills/software-development/python-debugpy/SKILL.md (+ zh-Hans mirror):
  "-p no:xdist"/"-n 0" pdb advice rewritten for the captured per-file
  subprocess runner.
- skills/creative/comfyui/tests/README.md: parent-repo "-n auto by
  default" rationale updated to past tense.

Combined salvage of PR #38295 (konsisumer), PR #51354 (TutkuEroglu,
redirected to the current conftest truth and the relocated
references/contributor-guide.md), and PR #54956 (waroffchange).

Co-authored-by: TutkuEroglu <rrandqua@gmail.com>
Co-authored-by: waroffchange <116298975+waroffchange@users.noreply.github.com>

AGENTS.md

@@ -1284,14 +1284,15 @@ def profile_env(tmp_path, monkeypatch):
 ### Python
 **ALWAYS use `scripts/run_tests.sh`** — do not call `pytest` directly. The script enforces
 hermetic environment parity with CI (unset credential vars, TZ=UTC, LANG=C.UTF-8,
-`-n auto` xdist workers, in-tree subprocess-isolation plugin). Direct `pytest`
+per-file subprocess isolation via `scripts/run_tests_parallel.py` — no xdist,
+worker count auto-scaled from CPU count). Direct `pytest`
 on a 16+ core developer machine with API keys set diverges from CI in ways
 that have caused multiple "works locally, fails in CI" incidents (and the reverse).
 
 ```bash
 scripts/run_tests.sh                                  # full suite, CI-parity
 scripts/run_tests.sh tests/gateway/                   # one directory
-scripts/run_tests.sh tests/agent/test_foo.py::test_x  # one test
+scripts/run_tests.sh tests/agent/test_foo.py -k test_x  # one test (file + -k; the runner is file-granular)
 scripts/run_tests.sh -v --tb=long                     # pass-through pytest flags
 ```