refactor: Eliminate unnecessary empty Vec constructions.

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

@@ -65,16 +65,15 @@ pub enum HookResponse {
 }
 
 /// Handler trait for lifecycle hooks. Implementations
-/// receive events and optionally return responses.
-/// Exec hooks return empty vecs. Handler hooks send
-/// responses back through the action channel asynchronously
-/// and also return empty vecs.
+/// receive events and may produce side effects (spawning
+/// processes, sending to channels). Responses flow through
+/// the action channel, not the return value.
 ///
 /// This is deliberately synchronous for dyn-compatibility.
 /// Implementations that need async work (spawning processes,
 /// writing to channels) should use `tokio::spawn` internally.
 pub trait HookHandler: Send + Sync {
-    fn handle(&self, event: HookEvent) -> Result<Vec<HookResponse>, PiklError>;
+    fn handle(&self, event: HookEvent) -> Result<(), PiklError>;
 }
 
 /// Parse a single line of JSON as a [`HookResponse`].