feat(core): Add runtime engine, input parsing, and hook trait.

crates/pikl-core/src/runtime/hook.rs

@@ -0,0 +1,15 @@
+//! Hook trait for lifecycle events. The core library defines
+//! the interface; concrete implementations (shell hooks, IPC
+//! hooks, etc.) live in frontend crates.
+
+use serde_json::Value;
+
+use crate::error::PiklError;
+
+/// A lifecycle hook that fires on menu events. Implementations
+/// live outside pikl-core (e.g. in the CLI binary) so the core
+/// library stays free of process/libc deps.
+#[allow(async_fn_in_trait)]
+pub trait Hook: Send + Sync {
+    async fn run(&self, value: &Value) -> Result<(), PiklError>;
+}