feat: introduce workflow management tools and commands

- Added new workflow commands: `/workflow` to open the workflow panel and `/workflow run <prompt>` to execute workflows.
- Implemented `spawn_agents` and `spawn_pipeline` tools for parallel and sequential task execution, respectively.
- Enhanced workflow engine to handle real-time agent status updates and display in the UI.
- Updated workflow panel to show agent statuses, findings count, and session counters.
- Refactored existing code to integrate new workflow functionalities and improve overall structure.

src/app/state/runtime.rs

@@ -103,6 +103,13 @@ pub enum TurnEvent {
     Compacted(Vec<crate::dto::chat::message::ChatMessage>),
     Error(String),
     Done,
+    /// Real-time update from a workflow subagent: push the new status
+    /// into `AppStateRest::workflow_engine.agents`.
+    WorkflowAgentUpdate {
+        agent_id: String,
+        agent_name: String,
+        status: crate::app::workflow::engine::AgentStatus,
+    },
 }
 
 impl SessionRuntime {