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/app.dart

@@ -119,6 +119,12 @@ class _RootShellState extends State<_RootShell> {
                 return null;
               },
             ),
+            QuickOpenIntent: CallbackAction<QuickOpenIntent>(
+              onInvoke: (_) {
+                widget.services.quickOpen.open();
+                return null;
+              },
+            ),
             FocusNextPanelIntent: CallbackAction<FocusNextPanelIntent>(
               onInvoke: (_) {
                 widget.services.focus.focusNextSlot();
@@ -150,6 +156,7 @@ class _RootShellState extends State<_RootShell> {
                           children: [
                             const Positioned.fill(child: RootLayout()),
                             const ClidePalette(),
+                            const QuickOpenOverlay(),
                             const Positioned.fill(child: _WelcomeOverlay()),
                           ],
                         ),