profiles: the Personal Secretary retires, and every reference goes with it ...

`secretary` had become a second general-purpose assistant: same models, same
tools, one temperature step apart from `assistant`, and admin-only for no
reason the user could see. The directory is gone and the eight remaining
profiles load clean.

A profile that no longer exists is not a harmless orphan — `Agent` resolves
`session.profile_id` on every run and raises `ProfileNotFound`, so any session
left behind would open and then fail on its first turn. Prod sessions were
moved to `assistant` first (115 rows, verified after), and the local clone
along with them (101).

The reaction fallback loses its role table: it existed because the admin
fallback was itself admin-only, which `assistant` never is. `spawn_agent`
falls back to `assistant` too, and the two prompts that recommended
`secretary` now name only `server_admin` and `developer`.

webclient: Backgrounds tab in artifacts panel + task toasts ...

- new Backgrounds tab: task list (running first, badges, per-tool icons)
  with a detail view (status, timestamps, sub-agent tokens, result/progress)
- GET /sessions/{id}/tasks snapshot endpoint: task_update events are not
  replayed on reconnect, so the client fetches the task list on session
  load/reload; live task_update entries are merged on top (chat.fetchTasks)
- terminal task_update no longer removes the entry — it marks it finished
  so the tab shows recent completions; _terminalTaskIds still blocks
  resurrection by a late running update
- toasts for background task start / finish (info / success / error) in
  the WS dispatch; silent for other sessions and unknown terminal ids
- persistent background-tasks chip removed (replaced by the tab); only
  the queued-messages chip stays
- fix invisible status text in terminal detail rows: filled .status-*
  backgrounds now scope to .terminal-status-badge only