feat(github): improve collaboration API parity

[SamyPesse]

GitHub-backed integration tests need collaboration and CI-status APIs, plus the same review-comment validation GitHub applies before changes are pushed.

• Add paginated user and installation-token repository discovery, stored reactions, and correct PR comment metadata. Installation-token listing intersects token selection with current grants and rejects removed or suspended installations.
• Validate review-comment diff locations, preserve reply locations, and emit pull_request.synchronize when PR heads advance.
• Support pending reviews with incremental GraphQL comments, submission/discard, author-only review and draft-comment reads, and submitted-summary edits. Filter repository-wide comments before pagination and keep that route reachable alongside pull-request reads.
• Add a GraphQL collaboration subset for paginated inline threads, resolve/unresolve, and draft/ready transitions, sharing REST state and emitting the corresponding public webhooks.
• Emit pull_request.edited with the updated base ref and SHA when a pull request is retargeted without other edits.
• Retain commit-status history with creation, newest-first paginated reads by SHA or ref, repository authorization, and signed status webhooks. Combined-status summaries remain outside this subset.
• Complete requested-reviewer lifecycle behavior: resolve users by durable ID, expose canonical user/team request lists, reject requesting the PR author, emit removal events only for stored requests, and clear a user's request on submitted reviews while drafts and team requests retain theirs.

Includes regression tests and documentation in the README, service skill, docs site, and CLI help. Draft edits and reaction changes do not invent public webhooks. Webhook receiver tests wait for delivery rather than dispatch admission.

Rebased onto upstream main 611f424 (v0.12.2). Validation: all 142 GitHub tests pass; core and GitHub package builds, GitHub typecheck, lint (existing warnings only), and changed-source formatting checks pass. The consuming OKC implementation passes its requested-reviewer journey and five existing GitHub review integration journeys, including user/group requests, cross-organization identity isolation, remote team removal, provider rejection preserving local requests, and submit-review reconciliation.

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