hive-runtime: replace the session when an ACP compact command fails

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

hive-runtime/README.md

@@ -52,3 +52,7 @@ past the watermark, or on `compact` (the operator's `/compact`, the agent's
   checkpoint turn first, as on claude.
 - Otherwise the policy's checkpoint turn runs, then the session is archived,
   and the next turn starts a new one, carrying the system prompt again.
+- A `compact` command that fails, or sends nothing for 180s (the turn's idle
+  window if that is shorter), is cancelled and handled as the case above; the
+  checkpoint turn is not run a second time. So a compaction always leaves a
+  smaller session behind, and a failed one is not retried on the next turn.