"Current task" block in the Plan page (activity + statuses + add-task + jump-to-chat)

### Goal
Make the Plan page usable as a standalone working surface by inserting a new **"Current task"** block in the middle. The top **Workflow/PR** cluster and the bottom **Done** block stay exactly as they are today — this issue only builds the middle block, and it's fully usable on its own.

### Scope
Insert a labeled **"Current task"** section between the Workflow/PR header and the Done block:
- **Live-activity readout** — the `in_progress` item plus a working / waiting / paused signal, driven by the existing `planGlance(isStreaming, askStates)` (never stored, so the agent can't fake it).
- **Transparent statuses** — clear per-item state glyphs/labels (working / waiting for answer / paused / pending / done) so the block honestly shows where each item stands, reusing the shared `planKit` status atoms.
- **Remaining pending items** listed below the current task, so what's next is visible.
- **Add task** footer row — queue a task from the plan with the chat closed (reuse `useChatTodos.add` + `nudgeAgent`/`shouldNudgeOnAdd`, which already avoids waking an agent mid-question; a `user`-origin loose item lands at the end). Optimistic add with an authority re-read on failure.
- **Jump-to-chat** (interim answer path) — when the state is **waiting for your answer**, the readout is clickable and opens/focuses the chat tab for that session, so "the agent needs me" is one tap away until Issue 3 lets you answer in place.

### Out of scope
Any change to the Workflow/PR header or the Done block; answering questions inside the plan (Issue 3).