fix(email): guard specification.html against generator drift

[github-actions]

The email agent's specification.html is a committed, integrator-facing doc that is a pure render of spec_html.py — but nothing stopped the two from drifting apart. If a hand-edited section had ever landed in the file without a matching generator change, the next render_endpoint_spec_html() regeneration would silently drop it and no test would notice. This adds the same drift guard openapi.email.json already has: a python -m gaia_agent_email.spec_html regenerate/--check CLI plus a test that fails loudly the moment the file and its generator diverge. (The pair is byte-identical today, so this is purely protective — no doc content changes.)

Closes #2080

Test plan

• python -m pytest tests/test_spec_html_artifact.py -x passes
• python util/lint.py --all passes
• python -m gaia_agent_email.spec_html --check exits 0 on the synced pair
• Append a stray line to specification.html → --check exits 1 and the drift test fails; revert → both pass again

🔍 Technical details

Root cause: specification.html had no regeneration path and no drift test, unlike its sibling artifact openapi.email.json (guarded by export_openapi.check_artifact() + test_committed_openapi_artifact_is_up_to_date). Regeneration was therefore unsafe — a file-only edit would survive until the next regen, then vanish silently.

Changes:

• gaia_agent_email/spec_html.py — add ARTIFACT_PATH (the committed specification.html next to openapi.email.json), write_artifact(), check_artifact() (reads the file, compares to a fresh render_endpoint_spec_html(), never rewrites), and a main() with --check/--output so python -m gaia_agent_email.spec_html regenerates and --check diffs. Exported in __all__; added if __name__ == "__main__". Directly mirrors export_openapi.py.
• tests/test_spec_html_artifact.py — new drift test mirroring the openapi one: asserts the committed file matches the generator, that a diverged/missing file is reported stale, and that write_artifact round-trips through check_artifact.

Acceptance criteria: the generator reproduces the committed file byte-identically (verified — check_artifact() returns True today); the new test fails loudly on divergence (verified by appending a line → --check exits 1); any generator-emitted section (incl. the referenced #1892/#1889 content, which is present in both today) now survives regeneration by construction, since the file is the generator output and the guard forbids file-only edits.

Verified: pytest tests/test_spec_html_artifact.py → 4 passed; tests/test_rest_contract.py → all pass except the pre-existing test_agent_version_matches_package_metadata (needs the wheel pip installed for dist metadata — an env artifact, not this change); black/isort/flake8 pass.