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/bindings/ContextRequirement.ts

@@ -0,0 +1,6 @@
+// This file was generated by [ts-rs](https://github.com/Aleph-Alpha/ts-rs). Do not edit this file manually.
+
+/**
+ * How strictly a context field is required.
+ */
+export type ContextRequirement = "not-required" | "optional" | "required" | "required-with-prompt";