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/research.rs

@@ -349,6 +349,12 @@ pub async fn create_topic(
     if !VALID_TOPOLOGY_KINDS.contains(&topology_kind) {
         return Err(ApiError::BadRequest);
     }
+    // Empty-roster gate: a workspace with zero agents cannot host research.
+    // The frontend already disables the create button; this closes the
+    // direct-POST hole so we never materialize an unstaffed topic.
+    if cm_db::repo::agents::count_active(&state.pool, user.workspace_id).await? == 0 {
+        return Err(ApiError::Conflict);
+    }
 
     // Ownership check before any writes: every agent must be in the caller's
     // workspace. Refuses to leak "agent exists" if it isn't visible.