Slice 5 — User can open a PR from a branch and merge it from the web UI

[safayavatsal]

What to build

The PR primitive — the smallest version that supports both human-authored and (eventually) Agent-authored PRs.

• Web UI: from a Repository, a "New Pull Request" flow lets the user pick a source and destination branch, see the diff, and submit. PR detail page shows the diff, a top-level comment field, and a "Merge" button.
• platform-api: POST /repos/:o/:n/pulls, GET /repos/:o/:n/pulls, GET /pulls/:id, POST /pulls/:id/comments, POST /pulls/:id/merge.
• GitRepository.Diff and GitRepository.Merge extended to support PR diff and merge-commit creation.
• On merge: destination branch advances, PR transitions to merged, a pr.merged event is published.
• On PR creation: a pr.opened event is published. (Reviewer Agent in slice 10 consumes this.)
• Comments stored in platform.pr_comments. Author is either a human user or, later, the Reviewer Agent (a special system actor).

Acceptance criteria

• A user can push a branch, open a PR against main, see the diff, and merge it.
• After merge, git clone reflects the merged state on main.
• PR list page shows open and closed PRs separately.
• A comment posted on a PR shows up immediately on reload.
• pr.opened and pr.merged events are emitted to the EventBus and observable in logs.
• Attempting to merge a PR with conflicts returns a clean error and does not corrupt the repo.

Blocked by

• Slice 4 — User can 'git push'; web file viewer renders the commit #4