docs: add beta release and rollback guide

[daichunghy]

Summary

Closes #5

Adds a release, upgrade, downgrade and rollback guide for the public beta Action. The guide pins the
full source commit, distinguishes annotated tags from commit SHAs and moving aliases, documents the
least-privilege shadow permissions, and states the remaining external-evidence gates.

It also adds a deterministic validator for the beta.5 release identity, full-SHA install path,
shadow permissions, rollback controls and non-claims.

Verification

• npm run verify
• 101 core tests
• 14 security tests
• 25 GitHub integration tests
• 6 CLI smoke tests
• npm run verify:dist
• npm run check:release-guide
• npm audit --audit-level=high: 0 vulnerabilities

Boundary

This documents the current public pre-release and does not claim Marketplace publication, stable
v0.1, live consumer installation, shadow pilot or adoption.

[daichunghy]

Verification update on this PR. The six required CI contexts, CodeQL, and the shadow check are green for faaca3b; the separate Review Gate is neutral by design for this shadow-only beta path. The PR remains blocked on the repository required independent approval, so there is no merge action for me to take.