Expose Git and PR actions in the UI #20

Expose commit, push, publish, merge, rebase, Create PR, PR review and state, and the lane's Git state. The shared Diff view supports CreatePrMenu, but the app does not wire the PR-creation callbacks, so no Create PR action is exposed yet.

Status

The channel details sidebar (#129) now shows the lane's branch, its changes (opening the Diff tab), and the newest PR from that branch with its state and checks. Actions ask the chat's agent, which lands the lane through lane push and gh pr create, so the UI never pushes around lane.

• Create PR and Create draft PR in details and the Diff header (feat(app): offer Create PR and fixing failed checks by asking the chat's agent #472)
• Fix failed checks from the chat's failed-checks line (feat(app): offer Create PR and fixing failed checks by asking the chat's agent #472)
• Commit (feat(app): offer commit, rebase, update, merge, and PR state actions by asking the chat's agent #475)
• Push / update an open PR (feat(app): offer commit, rebase, update, merge, and PR state actions by asking the chat's agent #475)
• Rebase onto the lane's base, with Changes.behind (feat(app): offer commit, rebase, update, merge, and PR state actions by asking the chat's agent #475)
• Merge, once checks and reviews allow and nothing is unpushed (feat(app): offer commit, rebase, update, merge, and PR state actions by asking the chat's agent #475)
• Ready for review, Close, and Address review when changes are requested (feat(app): offer commit, rebase, update, merge, and PR state actions by asking the chat's agent #475)
• Close PR when the PR can't merge yet (feat(app): offer git and PR actions in the Diff header, and Close for a PR that can't merge yet #478)
• Commit, update, rebase, and the PR state actions in the Diff header (feat(app): offer git and PR actions in the Diff header, and Close for a PR that can't merge yet #478)
• Exercise Commit, Create PR, Update PR, Merge, and Close end to end on a disposable PR (feat(app): offer git and PR actions in the Diff header, and Close for a PR that can't merge yet #478)
• Exercise Address review (needs a second reviewer to request changes) and Rebase end to end

Tracked by #6. Migrated from TODO.md — Port from old Ace.