hive-agent: dashboard Cancel and idle stalls for ACP agents

`/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

docs/web-ui/agent.md

@@ -244,8 +244,10 @@ Slash commands today:
 - `/help` — list commands locally.
 - `/clear` — wipe the local terminal view (server history kept).
 - `/cancel` — `POST /api/cancel` → host shellouts `pkill -INT
-  claude`, emits a Note. Also surfaces as a `■ cancel turn`
-  button in the state row while state=thinking.
+  claude`, emits a Note. On an ACP agent it sends the agent
+  `session/cancel` instead, and kills it if the turn hasn't ended 10s
+  later. Also surfaces as a `■ cancel turn` button in the state row
+  while state=thinking.
 - `/compact` — `POST /api/compact` → sets the deferred
   `Bus::request_compact()` flag and returns immediately. The harness
   runs the `/compact` at the next turn boundary (end of the in-flight