Add yaak-actions crate for centralized action system

Implements a unified action system that serves as a single source of truth
for all operations in Yaak (Tauri app, CLI, plugins, deep links, MCP server).

Key features:
- ActionExecutor: Combined registry and execution engine with async RwLock
- ActionHandler: Trait-based handlers using async closures
- Context system: RequiredContext and CurrentContext for action availability
- Action groups: Organize related actions
- TypeScript bindings: Auto-generated via ts-rs for frontend use

Design highlights:
- Handlers are closures (no dependencies on other yaak crates)
- Registration requires both metadata and handler (prevents orphan actions)
- Flexible return values via serde_json::Value
- All methods are async using tokio

All 33 tests passing. Ready for integration with yaak-core and yaak-app.

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

Cargo.lock

@@ -7994,6 +7994,17 @@ dependencies = [
  "rustix 1.0.7",
 ]
 
+[[package]]
+name = "yaak-actions"
+version = "0.1.0"
+dependencies = [
+ "serde",
+ "serde_json",
+ "thiserror 2.0.17",
+ "tokio",
+ "ts-rs",
+]
+
 [[package]]
 name = "yaak-app"
 version = "0.0.0"