fix: retire the pull request badge when the merge lands

[omartelo]

What

Merging a pull request left every badge around the Pulls screen still reading #N Open.

The footer badge, the session cards and the sidebar's "Pull request" entry only look the pull request up again when the checkout's HEAD moves, or when the window loses focus and gets it back. A merge does neither — the commit lands on the base branch, on the remote, and the worktree's HEAD never budges. So merging from the Pulls screen emptied that screen (onMerged refreshes it) while the badges went on showing the merged PR as open until you clicked away to another window and back.

How

lib/pulls/pull-request-lookup.ts — the shared lookup takes subscribers:

• invalidatePullRequests() clears the shared map and notifies. Clearing matters as much as the notify: inside the 2s SHARE_MS window a re-read would otherwise replay the pre-merge answer.
• onPullRequestInvalidated(reload) returns its unsubscribe; usePullRequest subscribes in the same effect that listens for focus.
• Pulls.tsx wraps refresh in a reload that invalidates first, and wires it to the three moments the state changes with HEAD standing still: the merge, the header's reload button, and a pull request opened from the empty state. The check poll and the focus re-read stay the screen's own — neither changes what a badge shows.

Every badge of every checkout is invalidated, not just the one that changed. A merge is rare and deliberate, and the callers of one checkout still collapse into a single gh call; the comment names the ceiling and the way out (key the listeners by path).

Not covered: gh pr merge run from a terminal next door. Nothing announces that, so it still waits for a window focus — same as before.

Test plan

• vitest — 408 passing, two new cases on the lookup: a re-read after an invalidation inside the share window, and subscribe/unsubscribe notification.
• biome check clean (12 pre-existing a11y warnings).
• tsc -b --noEmit + vite build.
• gofmt -l . + go vet ./... (no Go touched).
• By hand in task dev: merge a PR from the Pulls screen, confirm the footer badge, the session card #N and the sidebar's "Open" all clear without touching another window.