hive-runtime: compact ACP sessions through the agent's compact command

An ACP agent's session is now compacted like a claude one: proactively
once a turn crosses the percent-of-window watermark, and on the
operator's /compact or the agent's compact tool. Before, the ACP
backend's compact returned Unsupported and no watermark applied to it.

- AcpRuntime takes the same CompactionPolicy as ClaudeRuntime;
  make_session builds one PercentPolicy (with CHECKPOINT_PROMPT) and hands
  it to whichever backend runs.
- The runtime keeps the commands each session advertises in
  available_commands_update. If `compact` is among them, compaction sends
  the prompt `/compact` on the same session, which is how the ACP spec
  runs an advertised command. A proactive compaction runs the checkpoint
  turn first, as InfiniteSession does.
- With no compact command, the checkpoint turn runs, the session is
  archived, and the next turn starts a new one with the system prompt.
- Error::Unsupported had no producer left, so it and drive_turn's
  "/compact skipped" arm are gone.

Refs #4391

docs/turn-loop/claude-invocation.md

@@ -161,7 +161,8 @@ model-reported context fill reaches `HIVE_COMPACT_WATERMARK_PERCENT`
 for the window on turns the model didn't report one. `0` disables proactive
 compaction (the reactive path always applies). The proactive path is
 best-effort — a failed checkpoint or `/compact` never fails the turn that
-already succeeded.
+already succeeded. An ACP agent gets the same policy; how it compacts is in
+[`hive-runtime/README.md`](../../hive-runtime/README.md#acp-backend-compaction).
 
 The operator can force a compaction any time via `POST /api/compact`. It's
 **deferred**: the handler sets `Bus::request_compact()` and returns