rebuild the status model as herdr's state machine, event-driven and tested

the seen logic was inverted: first observations were marked done and real
completions idle, so the menu showed teal for agents that did nothing and
green for agents that finished, and attending changed nothing visible.

- watcher::advance is herdr's apply_pane_state_change + mark_active_tab_seen
  as a pure function, unit-tested against herdr's own cases; attendance is
  window-level like herdr's active tab
- hook reports are events applied in order, so a turn shorter than a poll is
  still a completion
- event-driven: hook reports, dingger focus, and tmux focus hooks wake the
  watcher; notifications are delivered off-thread (the macos call blocked
  ticks for 2+ seconds) after the pane options are written
- all per-pane state persists in tmux pane options, so restarts and deploys
  lose nothing, including the authoritative hook state
- every click goes through dingger focus: the menu bar shells out to it and
  the status-line binding runs it; it picks the right client, skips the
  window already shown, and activates the terminal app hosting the client
- the ready queue is ordered like the menu (blocked first, then oldest done)
- menu bar and tmux segment render herdr's symbols in herdr's palette
- tests/e2e.sh: 60 assertions against an isolated tmux server with a real
  client, a 30s poll interval, and a slow fake notifier

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