api: gate create_loop + create_topic on empty workspace roster (W2)

Frontend already disables the create button when the roster is empty,
but nothing stopped a direct POST from materializing an orphan loop /
topic with nothing to staff it. Both handlers now check the workspace
agent count up front and return 409 Conflict when it's zero.

- crates/cm-api/src/routes/loops.rs: gate at top of create_loop
- crates/cm-api/src/routes/research.rs: gate at top of create_topic

Uses the existing cm_db::repo::agents::count_active(pool, ws) helper
(count of workspace agents where deleted_at IS NULL). 409 is the right
mapping: state-of-the-workspace-prevents-this, not client-input-bad.

crates/cm-api/src/routes/loops.rs

@@ -114,6 +114,13 @@ pub async fn create_loop(
     if body.title.trim().is_empty() || body.task_template.trim().is_empty() {
         return Err(ApiError::BadRequest);
     }
+    // Empty-roster gate: a workspace with zero agents has nothing to staff
+    // the loop with — refuse before any DB writes. Frontend already
+    // disables the create button in this state; this closes the direct-POST
+    // hole so we don't materialize orphan loops that never fire.
+    if cm_db::repo::agents::count_active(&state.pool, user.workspace_id).await? == 0 {
+        return Err(ApiError::Conflict);
+    }
     let webhook_enabled = parse_triggers(&body.triggers)
         .map(|t| t.webhook_enabled)
         .unwrap_or(false);