Add a button to hand PR merge conflicts to the agent

[frenchie4111]

Summary

• The PR pane already knows when GitHub reports a conflicting merge, but acting on it meant switching to the chat and typing the request. This adds a one-click handoff: an Ask the agent to fix conflicts button appears above the CI-notify checkbox when hasConflict === true on an open PR.
• Clicking it injects a message naming the PR, branch, and base into that worktree's agent chat, reusing the existing deliverToWorktreeChat routing — so it wakes a slept chat tab the same way the CI-failure notifier does. It lands in the transcript as a labelled Ness · Merge conflicts card, not as something the user typed.
• Deliberately a button rather than the standing opt-in CI failures get: a branch that conflicts with its base usually conflicts with every other in-flight branch too, so firing this automatically would put the whole workspace to work over one bad merge base.

Two things the message deliberately leaves out, both commented at buildMergeConflictMessage:

• No file list. The local base ref is often stale, so a git merge-tree preview from Ness would name files the agent then finds clean — it has git and can see the real answer.
• No merge-vs-rebase prescription. Plenty of repos keep linear history. The agent can read the convention off git log and CLAUDE.md, and Ness has no setting that records it (mergeStrategy is how a PR lands on main, which says nothing about how a branch syncs with its base).

Test plan

• npm run typecheck clean
• npx electron-vite build clean
• npx vitest run — new merge-conflict-request.test.ts covers the sentinel round-trip (guards the merge-conflict automation source being registered, without which the chat would render it as a plain user turn) and that the PR number / branch / base / URL make it into the body
• Not visually verified in a running app — the button only renders when GitHub reports a conflicting PR, which I couldn't stage locally. Worth a manual look on a real conflicted PR before merge: button appearance, the sending → "Asked the agent" → idle transition, and the chat card rendering.

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