feat: enforce required agent roles at creation and role immutability in registry

.agents/skills/multi-agent-mux-create/SKILL.md

@@ -173,7 +173,7 @@ Use the `agent-sessions-yaml-edit` script in `scripts/` to safely append (preser
 
 ```bash
 bash .agents/skills/multi-agent-mux-create/scripts/create_session.sh \
-  --workspace "$WORKSPACE" --agent "$AGENT" --session "$SESSION_NAME"
+  --workspace "$WORKSPACE" --agent "$AGENT" --role "$ROLE" --session "$SESSION_NAME"
 ```
 
 The script handles the YAML append, pane capture, and the `last_visible_status` placeholder.