feat(tui): wire usePlanTree to DB indexer for live plan state

[paulbreuler]

Problem

src/tui/hooks/usePlanTree.ts has a stub implementation that always returns an empty plan array. The TUI Plans pane shows "No plans loaded" regardless of what's indexed.

Current behavior

export function usePlanTree(pollIntervalMs = 3000): PlanNode[] {
  const [plans, setPlans] = useState<PlanNode[]>([]);

  useEffect(() => {
    setPlans([]);
    const interval = setInterval(() => {
      // TODO: integrate with DB indexer
    }, pollIntervalMs);
    return () => clearInterval(interval);
  }, [pollIntervalMs]);

  return plans;
}

Expected behavior

The hook polls the DB indexer (or subscribes to file-watcher events) and returns a live tree of PlanNode[]. Each PlanNode contains id, name, status, and agents[] with real statuses from the agent frontmatter.

Acceptance criteria

• Hook fetches plans + agents from the SQLite DB on mount and on each poll interval
• PlanNode.agents reflects current agent statuses (GAP, WIP, PASS, BLOCKED)
• Updates are reflected within the pollIntervalMs window (default 3s)
• ACP update_status calls trigger a re-poll (via AcpSessionManager status_updated event)
• Hook cleans up interval on unmount (no memory leaks)
• Tests cover: empty workspace, multiple plans, status updates

Files to modify

• src/tui/hooks/usePlanTree.ts — implement DB polling
• src/tui/app.tsx — pass DB context or config to the hook
• tests/tui/hooks.test.ts — add tests with temp DB fixture

Related

• src/indexer.ts — listPlans, listAgents query functions
• src/acp/session-manager.ts — status_updated event to trigger re-poll