allow a max_subagents of 0: no subagents for the agent

hive-subagent-mcp/src/session.rs

@@ -2590,6 +2590,10 @@ pub fn interrupt(state: &State, name: &str) -> anyhow::Result<String> {
 fn refused(state: &State, name: &str, refusal: &Refusal, hint: &str) -> anyhow::Error {
     match refusal {
         Refusal::Taken => name_taken(state, name, hint),
+        Refusal::AtCap { cap: 0, .. } => anyhow::anyhow!(
+            "subagents are disabled for this agent: the cap is 0 (set in swarm-ui → this \
+             agent → inference)"
+        ),
         Refusal::AtCap { running, cap } => anyhow::anyhow!(
             "{running} {} running, cap is {cap} (set in swarm-ui → this agent → \
              inference) — wait for one to finish, or `interrupt` one",
@@ -4688,6 +4692,17 @@ mod tests {
         );
     }
 
+    #[test]
+    fn a_cap_of_zero_refuses_every_start() {
+        let state = State::new(PathBuf::from("/dev/null"), signal_url());
+        assert_eq!(
+            state.reserve_under("a", Some(0)),
+            Err(Refusal::AtCap { running: 0, cap: 0 })
+        );
+        let msg = refused(&state, "a", &Refusal::AtCap { running: 0, cap: 0 }, "").to_string();
+        assert!(msg.contains("disabled"), "{msg}");
+    }
+
     /// A run between turns and one winding down both still count.
     #[test]
     fn every_tracked_slot_counts_toward_the_cap() {