feat: add secure PR screenshot automation

[leoncheng57]

Summary

• parse strict fenced screenshots requests from PR descriptions
• capture the production SPA/BFF against deterministic Playwright mocks only
• validate untrusted artifacts before same-repository publication and sticky comments
• clean up only the closed PR's publication and marker-owned comment
• integrate the merged OpenCode 1.18.21 operations and expanded reminder catalogue

Security

The capture workflow is read-only and receives no secrets. A trusted default-branch workflow_run binds artifacts to the run SHA and publishes PNG bytes only when the PR head repository matches this repository. Fork artifacts are linked but never copied to gh-pages. The close workflow never checks out or executes PR code.

Verification

• npm run typecheck: client, server, and tools configs passed
• npm test: 21 files, 202 tests passed
• npm run build: production client and server passed
• npm run test:e2e: 121 passed, 1 screenshot-only placeholder skipped
• npm run screenshots:local: 2 PNGs captured and validated, including desktop and full:
• actionlint: all 4 workflows passed
• git diff --check: passed
• no mock/BFF ports remained after capture

The publisher is intentionally a default-branch workflow_run; this bootstrap PR proves capture via its artifact, and inline publication activates after merge.

/?directory=/tmp/mock-project
full:/sessions/ses_mock_done?directory=/tmp/mock-project