vim normal-mode navigation in non-editor panes (T-406)

The structural T-403 child: make vim normal mode mean navigation in panes
that were mouse-only. The passive global key path can't run multi-chord
sequences (D-82), so each pane hosts its own SequenceMatcher — factored into
a reusable PaneKeyNav that resolves the live keymap and dispatches nav.*
intents while a pane holds focus under the vim preset.

- nav.* intents (down/up/pageDown/pageUp/top/bottom/expandOrRight/
  collapseOrLeft/activate) — preset-neutral; vim.yaml binds j/k/ctrl+d/ctrl+u/
  gg/G/l/h/[o,enter] under `vim.normal && !editor.focused`.
- The editor publishes an `editor.focused` scope flag from its focus node, so
  the same keys stay buffer motions while the editor is focused and become nav
  when a pane is — resolved by file order + the guard (no change to the editor
  motion bindings).
- File tree: a flattened visible-index selection cursor in FileTreeController
  (j/k move, h collapse-or-out, l expand-or-into, o/enter open), with a focus
  ring + scroll-into-view.
- Conversation: j/k line-scroll, ctrl+d/u half-page, gg top, G bottom — G
  re-arms follow-tail.

Foundation for T-404/T-405/T-407, which build on the per-pane matcher and the
editor.focused guard. Git panel + ticket board list nav deferred to a
follow-up (the ticket says lists can trail). Tests: keymap resolution under
both scopes, PaneKeyNav dispatch, the controller selection model, and
end-to-end key-driven nav in both panes.

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

lib/builtin/editor/src/editor_view.dart

@@ -54,10 +54,16 @@ class _EditorViewState extends State<EditorView> {
     super.initState();
     _text = SyntaxTextController(syntax: _syntax);
     _focus = FocusNode();
+    _focus.addListener(_onFocusChanged);
     _text.addListener(_onTextChanged);
     _tabs.addListener(_onTabsChanged);
   }
 
+  /// Publish `editor.focused` so non-editor panes can guard their vim nav
+  /// bindings (`!editor.focused`) — when the editor holds focus, j/k/h/l/gg/G
+  /// stay buffer motions; when a pane holds focus they become nav (T-406).
+  void _onFocusChanged() => _keymap?.setScopeFlag('editor.focused', _focus.hasFocus);
+
   @override
   void didChangeDependencies() {
     super.didChangeDependencies();
@@ -75,12 +81,14 @@ class _EditorViewState extends State<EditorView> {
   void dispose() {
     _text.removeListener(_onTextChanged);
     _text.dispose();
+    _focus.removeListener(_onFocusChanged);
     _focus.dispose();
     _tabs.removeListener(_onTabsChanged);
     _tabs.dispose();
     _controller?.removeListener(_onControllerChanged);
     _controller?.dispose();
     _keymap?.removeListener(_onModeChanged);
+    _keymap?.clearScopeFlag('editor.focused');
     super.dispose();
   }