Planboard never moves an issue to Done: close it when the work lands #632

The board only ever moves forward. Start task (App.tsx:867) and autodispatch (autodispatch.js:45) call issues.setPlanStatus(..., "doing"); nothing in the app ever calls it with "done". The done edge is delegated to whichever agent happens to be looking — and the agent that does the work (a worker in its own worktree) is never the one that lands it, so the label stays at plan:doing forever. Nine issues were sitting in Doing today with all their code on main.

postmerge.js already has the inverse edge (#420 reopens the issue when post-merge verification fails), which only makes sense if something closes it.

Fix

issues.completeIssue(projectPath, number, {comment}) — gh issue view --json state,labels, then plan:done + gh issue close. Gated on the issue actually carrying plan:doing, so a stray #N in a prompt cannot close an issue nobody started. Already-closed is success.

postmerge.completeThreadIssue(store, threadId) resolves the thread's issue (thread.issueNumber → strict GitHub issue #N: prompt → loose issue #N in the first user message only) and fires it, once per issue per process.

Called fire-and-forget from the two places work actually lands:

• worktrees.mergeWorktree after a successful squash (the Git tab and thread_merge)
• postmerge.onThreadPrState on MERGED (interactive prStatus + the background PR refresher)

Leaves an event line on the thread: Planboard: #N moved to Done.

Not covered

An issue an agent flipped to plan:doing by hand for a thread whose work is landed outside the app (git merge in a terminal) still needs a human. A stale-Doing sweep would be a separate issue.