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).

crates/pikl-core/src/model/traits.rs

@@ -38,4 +38,25 @@ pub trait Menu: Send + 'static {
     /// Get the JSON value of a filtered item for output.
     /// Returns a reference to the stored value.
     fn serialize_filtered(&self, filtered_index: usize) -> Option<&serde_json::Value>;
+
+    /// Get the original index of a filtered item. Used to
+    /// provide the item's position in the unfiltered list
+    /// for output and hook events.
+    fn original_index(&self, filtered_index: usize) -> Option<usize>;
+
+    /// Replace all items with a new set of values. Used by
+    /// handler hook `replace_items` responses.
+    fn replace_all(&mut self, values: Vec<serde_json::Value>);
+
+    /// Remove items at the given original indices. Used by
+    /// handler hook `remove_items` responses.
+    fn remove_by_indices(&mut self, indices: Vec<usize>);
+
+    /// Get the formatted display text for a filtered item,
+    /// if a format template is configured. Returns None if
+    /// no template is set, in which case the raw label is
+    /// used.
+    fn formatted_label(&self, _filtered_index: usize) -> Option<String> {
+        None
+    }
 }