A `compact` command that errors, or sends nothing, used to leave the
session as full as before: `compacted` stayed false, so every following
turn ran the checkpoint and another `/compact` again, each one waiting
out the 600s turn idle window when the command was silent.
- The `/compact` turn gets its own idle bound, COMPACT_IDLE (3 min, or
the turn's idle window if shorter), through the turn's existing
watchdog, so a silent command is cancelled (or killed) like any
stalled turn.
- When the command fails or hits that bound, compaction falls back to
the no-command path: the session is archived and the next turn starts
a new one. The checkpoint turn runs there only if it has not already
run in this compaction.
- Compaction returns early when attaching produced a new session (a
failed `session/load` or a never-answered first prompt): there is
nothing in it to compact.
Refs #4391
b8ec0f4f85