feat: add pre-dispatch hook for agent task middleware

Generic extension point in community codebase: PreDispatchHook type
registered in AppState, checked before every agent_query dispatch.
Enables Pro features like budget enforcement and branch policy.

src-tauri/src/commands/agent.rs

@@ -9,6 +9,10 @@ pub fn agent_query(
     state: State<'_, AppState>,
     options: AgentQueryOptions,
 ) -> Result<(), String> {
+    // Run pre-dispatch hooks (budget enforcement, branch policy, etc.)
+    for hook in &state.pre_dispatch_hooks {
+        hook(&options)?;
+    }
     state.sidecar_manager.query(&options)
 }