mark as unread: make an idle agent done again, to come back to it

like "mark as unread" in mail: an agent you have seen (idle) becomes done
again, last among done agents of its importance, with no sound or
notification. it stays done until you come back to it: leave its tmux
window and return, or jump to it. marked while you look at it, it holds
until you look away (`held`); a new turn clears it. only idle agents can
be marked.

- daemon: `pane.mark_unread` on the hook socket; the watcher applies it
  after the tick's observations (mark_unread, advance's hold rule).
- `dingger unread [--pane]`: refuses done, working, blocked, and panes
  without an agent; returns once the agent shows as done.
- pi: /unread (refused while the agent works or is blocked).
- panel: right-click an idle agent in the full list, "Mark as unread".
- search: ⌘U or right-click marks the selected idle agent; the footer
  offers ⌘U when it applies; search stays open.

tests: unit (hold, jump, new turn, idle only), e2e step 22 (queue
position, hold, look away and back, jump, refusals, no sound, menu, the
extension), pi-live in real pi 1.4 (a mark, not a turn: an unknown
command would also end done), swift selftest (menu only for idle, footer).

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