add Vim mode service, mode commands, and status indicator

T-207, first foundation piece of the Vim layer (T-65 epic). A
VimModeService (ChangeNotifier) owns the normal/insert/visual mode and
mirrors it into the keymap as mutually-exclusive vim.normal/vim.insert/
vim.visual scope flags. Those flags are the public mode interface: the
editor (T-206) will read them to decide insert-vs-command, and vim.yaml
(T-65) guards bindings with `when: vim.*`. Nothing reaches across the
builtin boundary into the service object.

The layer is gated on the active preset — the builtin.vim extension
ties VimModeService.enabled to app.keymap.preset and re-checks on every
keymap reload, so i/v/Esc never hijack input under non-Vim presets. Mode
commands (vim.mode.{normal,insert,visual}) carry no default binding for
the same reason; only vim.yaml binds keys to them. A status-bar item
shows `-- NORMAL --` etc. while enabled.

Exposes KeymapService on the extension context so the layer can publish
scope flags.

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

lib/extension/src/extension.dart

@@ -10,6 +10,7 @@ import 'package:clide/kernel/src/files.dart';
 import 'package:clide/kernel/src/focus.dart';
 import 'package:clide/kernel/src/i18n/i18n.dart';
 import 'package:clide/kernel/src/ipc/client.dart';
+import 'package:clide/kernel/src/keymap/keymap_service.dart';
 import 'package:clide/kernel/src/log.dart';
 import 'package:clide/kernel/src/net.dart';
 import 'package:clide/kernel/src/notify.dart';
@@ -63,6 +64,7 @@ abstract class ClideExtensionContext {
   PanelRegistry get panels;
   LayoutArrangement get arrangement;
   CommandRegistry get commands;
+  KeymapService get keymap;
   PaletteController get palette;
   ReaderNavRegistry get readerNav;
   ClideClipboard get clipboard;