`/api/cancel` stopped a turn only by SIGINTing a child process whose argv0 is
`claude`, so on an ACP agent it reported "no claude process to interrupt"
and the turn ran on. The serve loop now stores the session's canceller on
the `Bus` when the runtime has one, and `/api/cancel` uses it: the agent is
sent `session/cancel`, and the next wake prompt carries the interrupted hint,
as for a signalled claude turn. Without a canceller (claude), the SIGINT path
is untouched.
An ACP turn stopped by the idle watchdog (`HIVE_TURN_IDLE_SECS`) becomes
`TurnError::AgentStall(note)`, handled like `ApiStall`: park for
`HIVE_STALL_SLEEP_SECS`, requeue, and record `api_stall`. The TurnEnd note
is the runtime's message (how long the agent was silent, whether it had to
be killed, and that a silently retried provider error such as HTTP 429
looks like this), not the claude-specific `ApiStall` text. A cancel the
agent ignores is a `Failed` turn.
Refs #4391
91e47732a6