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>

crates/yaak-actions/Cargo.toml

@@ -0,0 +1,15 @@
+[package]
+name = "yaak-actions"
+version = "0.1.0"
+edition = "2021"
+description = "Centralized action system for Yaak"
+
+[dependencies]
+serde = { workspace = true, features = ["derive"] }
+serde_json = { workspace = true }
+thiserror = { workspace = true }
+tokio = { workspace = true, features = ["sync"] }
+ts-rs = { workspace = true }
+
+[dev-dependencies]
+tokio = { workspace = true, features = ["rt-multi-thread", "macros"] }