feat(maintainers): add settings preview diagnostics

[Crystora]

Summary

Closes #21

Adds a maintainer-facing repo settings preview and install diagnostics endpoint.

Maintainers can now dry-run what the GitHub App would do for a sample PR before enabling public output: skip, comment, label, or create a minimal check run. The preview is read-only and does not mutate GitHub.

• Add POST /v1/repos/:owner/:repo/settings-preview.
• Return current repo settings, installation health, missing permissions/events, remediation guidance, dry-run decision, preview label/check-run state, and sanitized public comment preview.
• Add shared decidePublicSurface() logic and reuse it from the live webhook processor so preview behavior matches real PR handling.
• Cover confirmed miner, non-miner quiet skip, bot author, maintainer-author skip, disabled surface, missing permissions, missing events, invalid preview input, and unknown installation health.
• Keep check runs off by default unless explicitly enabled.
• Update OpenAPI and changelogs.

Validation

• npm run test:ci
• npm run changelog:check

Safety

• Backend-only change
• No secrets, wallet details, user PATs, raw trust scores, or private rankings exposed
• Public text avoids compensation-seeking or optimization-tactic language
• OpenAPI/MCP behavior updated where needed
• Public docs/changelogs updated where needed

[JSONbored]

PR #31 - feat: add repo settings preview and install diagnostics endpoint
Signal: REQUEST CHANGES

Required changes:

• Fix changelog freshness: CI fails at changelog:check.
• Rename PR title to conventional style, e.g. feat(maintainers): add settings preview diagnostics.
• Remove/regenerate packages/gittensory-mcp/CHANGELOG.md drift unless this PR truly changes the npm package.
• Add a test proving preview endpoint never mutates GitHub state.
• Add explicit test for label-only mode with missing Issues: write.
• Confirm settings-preview stays aligned with real webhook behavior after #29/#25 merge conflicts resolve.

[Crystora]

@JSONbored I've updated. pls review again. thanks