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/Cargo.toml

@@ -16,9 +16,8 @@ crate-type = ["staticlib", "cdylib", "rlib"]
 tauri-build = { version = "2", features = [] }
 
 [features]
-default = []
+default = ["multi-agent"]
 # Multi-agent orchestration (A2A protocol, Director, agent delegation)
-# Disabled by default — enable when multi-agent UI is ready.
 multi-agent = ["zclaw-kernel/multi-agent"]
 dev-server = ["dep:axum", "dep:tower-http"]