fix decision first-click + editor reveal via retained reader nav

Two reveal-on-open bugs:

Decisions opened only on the second click (T-196): the detail view
subscribed in didChangeDependencies, which runs after the tab is
revealed, so the broadcast 'selection' that triggered the reveal was
already gone. Hoist the back/forward history out of per-view State into
a retained per-reader ReaderNav (kernel ChangeNotifier in a
ReaderNavRegistry, D-81). The nav records selections, emits 'load' (the
single channel readers display from), and survives mount/unmount — the
reader grabs nav.current on mount, so the first selection lands. Both
the markdown and decisions readers move to this model; the per-view
ReaderHistoryMixin and the markdown post-frame forward hack are gone.

The editor pane never opened (T-197): EditorExtension contributed a
workspace tab but nothing activated it on editor.open. Add an activate()
that reveals the tab on editor.opened / editor.active-changed; the
view's hydrate() pulls the active buffer on mount.

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

lib/extension/src/extension.dart

@@ -2,6 +2,7 @@ import 'package:clide/extension/src/contribution.dart';
 import 'package:clide/kernel/src/clipboard.dart';
 import 'package:clide/kernel/src/commands/palette.dart';
 import 'package:clide/kernel/src/commands/registry.dart';
+import 'package:clide/kernel/src/reader_nav.dart';
 import 'package:clide/kernel/src/dialog.dart';
 import 'package:clide/kernel/src/events/bus.dart';
 import 'package:clide/kernel/src/events/message_bus.dart';
@@ -63,6 +64,7 @@ abstract class ClideExtensionContext {
   LayoutArrangement get arrangement;
   CommandRegistry get commands;
   PaletteController get palette;
+  ReaderNavRegistry get readerNav;
   ClideClipboard get clipboard;
   FileServices get files;
   Notifications get notify;