Complete Public Preview pre-visibility validation

[tensornull]

Summary

• update the credential-free workflow actions covered by Dependabot PRs chore(deps): bump actions/download-artifact from 4.3.0 to 8.0.1 #1, chore(deps): bump actions/checkout from 4.2.2 to 7.0.1 #2, chore(deps): bump actions/upload-artifact from 4.6.2 to 7.0.1 #4, and chore(deps): bump actions/setup-python from 5.6.0 to 7.0.0 #6 to their reviewed SHA-pinned releases
• retain package artifacts outside the copied-checkout candidate and recheck their SHA256 digests
• replace text-fragment workflow checks with parsed YAML contracts for triggers, permissions, runners, environments, exact commands, action identities, inputs, and release-chain ordering
• add hostile regression coverage for trigger filters, secret-context access, failure/no-op decoys, environment overrides, arbitrary OIDC actions, and mutable action references
• keep .github/CODEOWNERS absent through the Public Preview content gate

Dependency PR disposition

• this PR is intended to supersede chore(deps): bump actions/download-artifact from 4.3.0 to 8.0.1 #1, chore(deps): bump actions/checkout from 4.2.2 to 7.0.1 #2, chore(deps): bump actions/upload-artifact from 4.6.2 to 7.0.1 #4, and chore(deps): bump actions/setup-python from 5.6.0 to 7.0.0 #6 after its current-base credential-free CI succeeds
• chore(deps): bump pypa/gh-action-pypi-publish from 1.14.0 to 1.14.1 #3 and chore(deps): bump googleapis/release-please-action from 4.4.1 to 5.0.0 #5 remain deferred because their real publish/release paths cannot be exercised by credential-free pull-request CI; they stay out of main

Local verification

• uv sync --locked
• Ruff lint and format checks
• Pyright
• 173 offline tests passed; the separately marked live test was not run
• version/changelog and Public Preview document gates
• secret and workflow semantic checks
• clean wheel and sdist build, Twine metadata, artifact identity, SHA256, and independent clean-install smokes
• copied-checkout repository-independence gate, including its full nested build/install suite
• actionlint 1.7.12 online and offline
• independent 76-case hostile workflow review with no accepted hostile case

Scope boundary

This PR does not change repository visibility, configure secrets or environments, make live API calls, create tags or releases, publish to PyPI, or change repository protection settings.