hive-subagent-mcp: fix build against AcpRuntime's new policy parameter

#4822 added a CompactionPolicy generic parameter and a policy argument
to AcpRuntime::new. #4822 and #4824 (which added acp_runtime's call)
were reviewed and CI'd independently and merged textually clean, so
neither build caught that the two conflict: forge/main fails to
compile with 'missing generics for struct AcpRuntime' and 'this
function takes 4 arguments but 3 were supplied'.

Parameterize AcpRuntime<hive_claude::NeverCompact>, matching this
crate's documented no-mid-turn-compaction design (session.rs's module
doc: subagents are bounded, single-batch work, not sessions long-lived
enough to need in-place compaction).

hive-subagent-mcp/src/session.rs

@@ -1696,7 +1696,7 @@ async fn after_turn(
 fn spawn_and_track_acp(
     state: &Arc<State>,
     name: &str,
-    runtime: AcpRuntime,
+    runtime: AcpRuntime<hive_claude::NeverCompact>,
     config: Config,
     prompt: String,
     verdict: Option<VerdictTx>,
@@ -1743,11 +1743,20 @@ fn acp_session_file(sessions: &Path, name: &str) -> PathBuf {
 /// The ACP runtime `name`'s run drives, keeping its session id in
 /// [`acp_session_file`] and answering permission requests per
 /// [`acp_permits`].
-fn acp_runtime(command: AcpCommand, sessions: &Path, name: &str) -> AcpRuntime {
+fn acp_runtime(
+    command: AcpCommand,
+    sessions: &Path,
+    name: &str,
+) -> AcpRuntime<hive_claude::NeverCompact> {
     let groups = hive_sh4re::permissions::effective_tool_groups();
     let permit: PermissionPolicy =
         Arc::new(move |ask: &PermissionAsk<'_>| acp_permits(ask, &groups));
-    AcpRuntime::new(command, acp_session_file(sessions, name), permit)
+    AcpRuntime::new(
+        command,
+        acp_session_file(sessions, name),
+        permit,
+        hive_claude::NeverCompact,
+    )
 }
 
 /// Whether an ACP subagent may run the tool call it asks about.