docs: fix 41 unresolved dartdoc references

Convert non-resolving [refs] in doc comments to backtick code-spans across 24
lib/ files (param/field names out of doc scope, method refs on other classes,
non-API strings like regex char-classes and command ids). Verified 0
"unresolved doc reference" warnings via `dart doc --validate-links`. The
dart-doc CI gate (test.yml) never ran before — Gitea Actions was inactive — so
this debt had accumulated unchecked.

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

lib/builtin/vim/src/vim_mode_service.dart

@@ -9,7 +9,7 @@
 /// the `vim.yaml` preset (T-65) guards its bindings with `when: vim.normal`
 /// etc. Nothing reaches into this object across the builtin boundary.
 ///
-/// The whole layer is gated by [enabled], which the Vim extension ties to
+/// The whole layer is gated by `enabled`, which the Vim extension ties to
 /// the active preset: under a non-Vim preset the flags are cleared so they
 /// can never affect another preset's bindings.
 library;
@@ -47,7 +47,7 @@ class VimModeService extends ChangeNotifier {
   /// Whether the Vim layer is live. False under non-Vim presets.
   bool get enabled => _enabled;
 
-  /// The active mode. Meaningful only while [enabled]; defaults to
+  /// The active mode. Meaningful only while `enabled`; defaults to
   /// [VimMode.normal] and resets to it whenever the layer is enabled.
   VimMode get mode => _mode;