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/src/recent_files.dart

@@ -0,0 +1,35 @@
+/// A bounded, most-recent-first list of repo-relative file paths opened
+/// this session. Backs the quick-open overlay's empty-query state
+/// (T-51). In-memory only — recents reset per app run, matching the
+/// session-scoped "recently opened within a workspace" convention.
+library;
+
+import 'package:flutter/foundation.dart';
+
+class RecentFilesService extends ChangeNotifier {
+  RecentFilesService({this.cap = 20});
+
+  /// Maximum number of paths retained; the oldest fall off the end.
+  final int cap;
+
+  final List<String> _paths = [];
+
+  /// Most-recent-first snapshot of the retained paths.
+  List<String> get paths => List.unmodifiable(_paths);
+
+  /// Record [path] as the most-recently opened file: moves an existing
+  /// entry to the front (no duplicates) and trims to [cap].
+  void push(String path) {
+    if (path.isEmpty) return;
+    _paths.remove(path);
+    _paths.insert(0, path);
+    if (_paths.length > cap) _paths.removeRange(cap, _paths.length);
+    notifyListeners();
+  }
+
+  void clear() {
+    if (_paths.isEmpty) return;
+    _paths.clear();
+    notifyListeners();
+  }
+}