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

hive-runtime/README.md

@@ -40,6 +40,15 @@ with `AcpError::IdleTimeout` (`IdleKilled` if the agent was killed). This is
 the only way a provider error the agent retries without reporting it, such as
 an HTTP 429, ends the turn.
 
-## ACP backend: not yet
+## ACP backend: compaction
 
-`compact` returns `Error::Unsupported`.
+The same `CompactionPolicy` as the claude backend decides when: after a turn
+past the watermark, or on `compact` (the operator's `/compact`, the agent's
+`compact` tool).
+
+- If the agent advertises a `compact` command (`available_commands_update`),
+  it is sent as the prompt `/compact` on the same session, as the ACP spec
+  runs any advertised command. A proactive compaction runs the policy'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.