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/DEVPLAN.md

@@ -14,7 +14,7 @@ when* we build it.
 - Ship something usable early, iterate from real usage
 - Don't optimize until there's a reason to
 
-## Phase 1: Core Loop (TUI)
+## Phase 1: Core Loop (TUI) ✓
 
 The minimum thing that works end-to-end.
 
@@ -35,7 +35,7 @@ The minimum thing that works end-to-end.
 **Done when:** `ls | pikl` works and prints the selected
 item.
 
-## Phase 1.5: Action-fd (Headless Mode)
+## Phase 1.5: Action-fd (Headless Mode) ✓
 
 Scriptable, non-interactive mode for integration tests and
 automation. Small enough to slot in before phase 2. It's
@@ -60,7 +60,7 @@ for `show-ui`.
 **Done when:** `echo -e "hello\nworld" | pikl --action-fd 3`
 with `confirm` on fd 3 prints `"hello"` to stdout.
 
-## Phase 2: Navigation & Filtering
+## Phase 2: Navigation & Filtering ✓
 
 Make it feel like home for a vim user. The filter system
 is the real star here: strategy prefixes, pipeline
@@ -112,7 +112,7 @@ chaining, incremental caching.
 with vim muscle memory.
 `'log | !temp | /[0-9]+/` works as a pipeline.
 
-## Phase 3: Structured I/O & Hooks
+## Phase 3: Structured I/O & Hooks ✓
 
 The structured data pipeline and the full hook system.