subagent: add status tool, cut docs down to operator-facing + no cli flags

hive-subagent-mcp/src/session.rs

@@ -235,6 +235,30 @@ fn spawn_and_track(
     Ok(format!("subagent `{name}` started"))
 }
 
+/// Report whether `name` is currently running — a zero-cost check that
+/// never launches a process, unlike `continue`. Distinguishes three
+/// states: running, idle (a session exists but nothing is in flight), and
+/// no such session at all.
+///
+/// # Errors
+///
+/// An invalid name, or no session — running or on disk — under `name`.
+pub fn status(state: &State, name: &str) -> anyhow::Result<String> {
+    validate_name(name)?;
+    if state.is_running(name) {
+        return Ok(format!("subagent `{name}` is running"));
+    }
+    let config = build_config(name, None, None);
+    let store = build_store(&config)?;
+    if store.find_by_title(name).is_some() {
+        Ok(format!(
+            "subagent `{name}` is idle — its last turn finished; `continue` to give it another"
+        ))
+    } else {
+        anyhow::bail!("no subagent named `{name}` exists — `start` creates one")
+    }
+}
+
 /// Signal `name`'s running process — `force` picks SIGKILL over SIGINT (see
 /// `hive_claude::Cancel::cancel`). Refuses a name with nothing running:
 /// there's no queued/pending state to cancel pre-emptively any more (see