Live PR review comments in-app, one click to hand to the worktree's agent

[liam-russell]

Problem

When a PR receives review comments (human reviewers, bots, CI annotations), the loop leaves the app: notice the comment on github.com, copy the context into the agent's terminal, paste. Worker agents can't respond to live feedback at all.

Proposal

• Poll review threads + issue comments for the worktree's PR; show a feed on the worktree with an unresolved-count badge in the sidebar.
• "Send to agent": inject the comment — with file/line context — into the worktree's ACP chat session (chat:send) or agent terminal as a formatted prompt.
• Optionally let the agent reply to / resolve the thread via the provider API, gated by the same per-workspace agent-permissions setting as the PR-lifecycle work.
• New-comment events feed the same notification surface as OS notification when a background agent session finishes #92.

Even with no agent involved this is valuable: review feedback appears next to the diff instead of in a browser tab.

Depends on: #138 (PR-lifecycle provider work). Part of the #134 orchestration story: workers keep their PRs moving in response to feedback without the user shuttling context.