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/kernel/src/extensions_manager.dart

@@ -5,6 +5,7 @@ import 'package:clide/extension/src/extension.dart';
 import 'package:clide/kernel/src/clipboard.dart';
 import 'package:clide/kernel/src/commands/keybindings.dart';
 import 'package:clide/kernel/src/commands/palette.dart';
+import 'package:clide/kernel/src/reader_nav.dart';
 import 'package:clide/kernel/src/commands/registry.dart';
 import 'package:clide/kernel/src/dialog.dart';
 import 'package:clide/kernel/src/events/bus.dart';
@@ -40,6 +41,7 @@ class ExtensionManager extends ChangeNotifier {
     required this.arrangement,
     required this.commands,
     required this.palette,
+    required this.readerNav,
     required this.keybindings,
     required this.keymap,
     required this.clipboard,
@@ -65,6 +67,7 @@ class ExtensionManager extends ChangeNotifier {
   final LayoutArrangement arrangement;
   final CommandRegistry commands;
   final PaletteController palette;
+  final ReaderNavRegistry readerNav;
   final KeybindingResolver keybindings;
   final KeymapService keymap;
   final ClideClipboard clipboard;
@@ -289,6 +292,8 @@ class _ExtensionContext implements ClideExtensionContext {
   @override
   PaletteController get palette => manager.palette;
   @override
+  ReaderNavRegistry get readerNav => manager.readerNav;
+  @override
   ClideClipboard get clipboard => manager.clipboard;
   @override
   FileServices get files => manager.files;