feat(multi-agent): enable Director + butler delegation (Chunk 4)

- Enable multi-agent feature by default in desktop build
- Add butler delegation logic: task decomposition, expert assignment
- Add ExpertTask, DelegationResult, butler_delegate() to Director
- Add butler_delegate_task Tauri command bridging Director to frontend
- 13 Director tests passing (6 original + 7 new butler tests)

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

desktop/src-tauri/src/lib.rs

@@ -193,6 +193,8 @@ pub fn run() {
             kernel_commands::a2a::agent_a2a_discover,
             #[cfg(feature = "multi-agent")]
             kernel_commands::a2a::agent_a2a_delegate_task,
+            #[cfg(feature = "multi-agent")]
+            kernel_commands::a2a::butler_delegate_task,
 
             // Pipeline commands (DSL-based workflows)
             pipeline_commands::discovery::pipeline_list,