feat: Expand hook system to handle simple exec and plugin extensibility.

Item Model Expansion - Item now caches sublabel, icon, group with accessors. Added resolve_field_path() for dotted path traversal and field_value() on Item.
Output Struct - New OutputItem with OutputAction (select/cancel) and index. Object values flatten, strings get a value field. MenuResult::Selected now carries { value, index }.
Hook Types - Replaced the old Hook trait with HookEvent (serializable, 6 variants), HookResponse (deserializable, 5 commands), HookHandler trait (sync for dyn-compatibility), and parse_hook_response() with tracing warnings.
New Actions & Menu Methods - Added ReplaceItems, RemoveItems, ProcessHookResponse, CloseMenu actions. Menu trait gained original_index(), replace_all(), remove_by_indices(), formatted_label(). Pipeline got rebuild() and rebuild_with_values(). Smart cursor preservation on replace.
Lifecycle Events - MenuRunner emits Open, Close, Hover, Select, Cancel, Filter events through the dispatcher. Cursor tracking for Hover detection.
Debounce - DebouncedDispatcher with 4 modes: None, Debounce, CancelStale, DebounceAndCancelStale. Defaults: hover=DebounceAndCancelStale(200ms), filter=Debounce(200ms).
Exec Hooks - ShellExecHandler maps --on-{open,close,hover,select,cancel,filter}-exec flags to fire-and-forget subprocesses. Event JSON piped to stdin.
Handler Hooks - ShellHandlerHook launches persistent processes per --on-{event} flag. Bidirectional JSON lines: events on stdin, responses on stdout flowing back through Action::ProcessHookResponse. CompositeHookHandler dispatches to both.
--filter-fields - --filter-fields label,sublabel,meta.tags searches multiple fields. Combined text for fuzzy, individual for exact/regex.
--format - FormatTemplate parses {field.path} placeholders. --format '{label} - {sublabel}' controls display. TUI renders formatted_text when available.
Field Filters - meta.res:3840 in query syntax matches specific fields. !meta.res:3840 for inverse. Pipeline stores item Values for field resolution. Requires dotted path (single word colons stay fuzzy).

docs/DESIGN.md

@@ -213,17 +213,24 @@ them. The core manages debouncing and cancel-stale logic
 since that interacts with the event loop timing.
 
 ```rust
-#[async_trait]
 pub trait HookHandler: Send + Sync {
-    async fn handle(&self, event: HookEvent)
+    fn handle(&self, event: HookEvent)
         -> Result<Vec<HookResponse>, PiklError>;
 }
 ```
 
-The CLI binary provides `ShellHookHandler`, which maps
-CLI flags to shell commands. Library consumers implement
-their own handlers with whatever behaviour they want:
-in-process closures, network calls, anything.
+The trait is deliberately synchronous for dyn-compatibility.
+Implementations that need async work (spawning processes,
+writing to channels) use `tokio::spawn` internally. This
+keeps the trait object-safe so the core can hold
+`Arc<dyn HookHandler>`.
+
+The CLI binary provides `ShellExecHandler` and
+`ShellHandlerHook`, which map CLI flags to shell commands.
+A `CompositeHookHandler` dispatches to both based on event
+kind. Library consumers implement their own handlers with
+whatever behaviour they want: in-process closures, network
+calls, anything.
 
 ## Filtering