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/src/acp/stream.rs

@@ -185,6 +185,23 @@ pub(super) fn session_model(response: &Value) -> Option<String> {
         .map(str::to_owned)
 }
 
+/// The command names an `available_commands_update` advertises, or `None`
+/// for any other `update`. Each replaces the session's previous list.
+pub(super) fn advertised_commands(update: &Value) -> Option<Vec<String>> {
+    if update.get("sessionUpdate").and_then(Value::as_str) != Some("available_commands_update") {
+        return None;
+    }
+    let commands = update.get("availableCommands").and_then(Value::as_array);
+    Some(
+        commands
+            .into_iter()
+            .flatten()
+            .filter_map(|c| c.get("name").and_then(Value::as_str))
+            .map(str::to_owned)
+            .collect(),
+    )
+}
+
 /// Convert a claude `--mcp-config` document into ACP's `mcpServers` list.
 /// Returns the list and the server names, in the same order.
 pub(super) fn mcp_servers(config: &Value) -> (Vec<Value>, Vec<String>) {