add quick-open fuzzy file finder (Ctrl/Cmd+P)

A file picker overlay over the whole workspace, distinct from the
command palette. QuickOpenController holds the file list + a
subsequence fuzzy filter; the overlay loads the list via files.walk on
open, shows RecentFilesService entries on an empty query, and opens the
selection through a shared openWorkspaceFile helper (.md → markdown
reader bus, else editor.open) that the files panel now also routes
through, so recents stay in sync from every open site.

Bound to ctrl+p / meta+p with `when: !palette.open` so it never
collides with the palette's ctrl+p navigation; in-overlay arrows/enter/
escape reuse the palette's keymap-driven model via quickOpen.* intents.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

lib/kernel/kernel.dart

@@ -29,6 +29,7 @@ export 'src/keymap/keymap_service.dart';
 export 'src/keymap/when_clause.dart';
 export 'src/dialog.dart';
 export 'src/extensions_manager.dart';
+export 'src/file_open.dart';
 export 'src/files.dart';
 export 'src/focus.dart';
 export 'src/i18n/catalog_loader.dart';
@@ -39,6 +40,8 @@ export 'src/notify.dart';
 export 'src/os.dart';
 export 'src/panels/arrangement.dart';
 export 'src/project.dart';
+export 'src/quick_open.dart';
+export 'src/recent_files.dart';
 export 'src/scheduler.dart';
 export 'src/text_zoom.dart';
 export 'src/secrets.dart';