Show the pull request in PR-watch monitor rows

[voida6]

When a thread watches pull requests, each watch shows up as background work labelled only "Watching pull request #20051". To see which PR it is, or whether its checks are passing, you have to leave the thread and look it up.

The thread already has everything needed. PR-watch monitors are built from the thread's linked pull requests (pullRequestWatchTasks in packages/shared/src/orchestrationV2PendingBackgroundWork.ts). Clients already render linked PRs with title, state, checks and review (LinkedPullRequestList on mobile). The monitor just drops that link and keeps a text label.

Proposal

• Keep a reference to the linked PR on PR-watch monitor tasks: an optional pullRequest key on the monitor variant in contracts, filled in by pullRequestWatchTasks.
• In the background-work list (the web banner and the mobile sheet from feat(mobile): tap the background work pill to see what's running #21346), show those rows as the PR: title, number coloured by state (open, draft, merged, closed), and check status.
• Tapping a row opens the PR on GitHub.

Out of scope: other monitor types, per-monitor stop or unwatch, and "last woke" timing. Those can be separate discussions.

Would this direction be welcome? If so, I'll build it on top of #21346 once that lands.