Implement PR, push, and commit review triggers

[romgenie]

Local source: coven-github/issues/07-implement-pr-push-and-commit-review-triggers.md

Summary

The reimagined GitHub App product includes automatic review on @mention, commit, PR, or any configured combination. coven-github currently implements issue assignment, issue label, issue mention, and PR review-comment mention paths, but not PR lifecycle or push/commit review lanes.

Current Evidence

• README.md lists V1 triggers as issue assigned to bot user, coven: label applied to issue, @cody mention in issue comment, and PR review comment @cody fix:.
• crates/webhook/src/events.rs parses issues assigned/labeled, issue comments, and pull request review comments.
• The revised architecture diagram marks PR opened/synchronize and commit/push review as target trigger lanes rather than implemented lanes.
• docs/hosted-mvp-plan.md starts with issue/label triggers but the broader product direction includes hosted review and codebase-aware GitHub work.

Problem

Without PR and commit trigger support, the GitHub App cannot function as a general code-review agent. It is primarily an issue-to-PR agent plus manual mention responder.

Impact

• Teams cannot install it as an automatic reviewer on every PR.
• Commit/push review and branch-range review are unavailable.
• Combined workflows cannot work, for example automatic shallow review on PR open plus @familiar deepen for targeted re-review.
• Product messaging around code review is ahead of the actual event coverage.

Proposed Design

Add a trigger policy layer that supports:

• mention: current issue/PR comment command behavior;
• issue_assignment: current issue assignment behavior;
• issue_label: current label behavior;
• pull_request: opened, synchronize, reopened, ready_for_review, labeled;
• push: branch pushes and commit range review;
• manual: Cave/dashboard/API initiated task.

Each trigger should map to a typed task kind with immutable target refs. Repository policy should decide which triggers are enabled, which branches/paths are in scope, and whether the task is advisory, blocking, or fix-capable.

Acceptance Criteria

• Event parser supports pull_request and push payloads with fixture tests.
• Router can enable/disable each trigger by repo/familiar policy.
• PR synchronize events coalesce or supersede older queued work for the same PR/head.
• Push events can review a single commit or compare range.
• Mention commands can re-run, deepen, or request a fix for an existing PR task.
• Docs and README accurately distinguish implemented trigger lanes from planned lanes.

Test Notes

Add GitHub webhook fixture tests for PR opened, PR synchronize, PR ready_for_review, push to default branch, push to feature branch, and ignored branch. Verify task kind, target SHA, branch, repo, installation, and familiar routing.