When enabled via `[tui] pinned_plan_drawer = true`, plan updates render
in a fixed viewport drawer between the active cell and the bottom pane
instead of scrolling away as history cells. The drawer updates in place
on each PlanUpdate event, keeping the latest plan always visible.

- Add `pinned_plan_drawer` config field (default: false) with TOML
  persistence and /config menu toggle
- Extract `render_plan_lines()` shared function from PlanUpdateCell
  for reuse by both history cells and the new PinnedPlanDrawer widget
- Modify `on_plan_update()` to route to drawer or history based on config
- Re-propagate setting across /new, /fork, agent switch, and loop reset
🤖 Generated with [Nori](https://usenori.ai)

Co-Authored-By: Nori <contact@tilework.tech>

…ersation

Previously, toggling the pinned plan drawer on mid-conversation showed
nothing because plan updates received while the drawer was disabled were
only routed to history cells. Now `pinned_plan` always tracks the latest
plan update regardless of drawer state, and rendering is gated on the
`pinned_plan_drawer` boolean. Re-enabling the drawer after disabling it
also preserves the last plan.
🤖 Generated with [Nori](https://usenori.ai)

Co-Authored-By: Nori <contact@tilework.tech>