feat(tasks): auto-compact a resumed session before the round's prompt

Task settings gain an auto-compact threshold (percent, 0 = off) and an
optional compact command. When a retry, follow-up or merge round resumes
a session whose context window is at least that full, the engine sends
the compact command, waits for that turn to land, and only then sends
the round's own message; a fresh session is never compacted. Occupancy
comes from the live connection's usage, falling back to the parsed
transcript only when it belongs to the session actually connected. The
command resolves per agent — the setting, then a compaction verb the
session advertises, then a built-in default — and every outcome is
recorded on the task timeline with the before/after occupancy.

The compaction turn is fenced off from the round it precedes by
(connection, event seq) on every exit of the wait: the internal bus is a
broadcast, so the engine's own receiver can dequeue that completion long
after the launch has bound the connection to the run, and would settle
the round and disconnect the agent mid-work. The wait also polls
liveness, because apply_event clears turn_in_flight on TurnComplete but
not on a terminal error or a disconnect, so a lagged terminal event
would park the launch forever holding the task lock.