implement builtin.files — workspace file tree with live watcher

Flutter sidebar tab that lazy-loads the workspace tree via IPC
files.ls and refreshes subtrees on files.changed events. Click-to-
open routes through a future editor.open command; until Tier 2
registers it, the execute call no-ops gracefully.

Daemon side adds a new files subsystem:
  - files.root returns the resolved workspace root (git root if
    present, otherwise cwd)
  - files.ls lists a directory with ignore filtering applied
  - files.watch starts a recursive Directory.watch and fans
    FileSystemEvents out as files.changed IPC events
  - FilesService owns the watcher + ignore set lifecycle

IgnoreSet + IgnorePattern implement the common gitignore subset:
anchored (/foo), directory-only (foo/), negation (!foo), **
crossing dirs, ** at trailing position. Built-in layer hides clide-
owned dirs (.git, .pql, .clide, .dart_tool, build, node_modules);
.gitignore + .clideignore at the root layer on top per D-004. Full
multi-file ignore_files: layering from .pql/config.yaml is future
work.

11 new ignore-matcher tests + 5 files.* dispatcher tests.

Co-Authored-By: Claude <noreply@anthropic.com>

bin/clide.dart

@@ -72,9 +72,13 @@ Future<void> _runDaemon(List<String> args) async {
     socketPath: socketPath,
     dispatch: dispatcher.dispatch,
   );
-  final registry = PaneRegistry(events: _ServerEventSink(server));
+  final events = _ServerEventSink(server);
+  final registry = PaneRegistry(events: events);
   registerPaneCommands(dispatcher, registry);
 
+  final files = FilesService.atCwd(events: events);
+  registerFilesCommands(dispatcher, files);
+
   final stopping = Completer<void>();
   void shutdown(ProcessSignal sig) {
     if (!stopping.isCompleted) {
@@ -89,6 +93,7 @@ Future<void> _runDaemon(List<String> args) async {
   await server.start();
   await stopping.future;
   await registry.shutdown();
+  await files.shutdown();
   await server.stop();
   exit(0);
 }