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/builtin/markdown/src/extension.dart

@@ -4,7 +4,6 @@ import 'package:clide/builtin/markdown/src/markdown_viewer.dart';
 import 'package:clide/extension/extension.dart';
 import 'package:clide/kernel/kernel.dart';
 import 'package:clide/widgets/widgets.dart';
-import 'package:flutter/widgets.dart';
 
 class MarkdownExtension extends ClideExtension {
   @override
@@ -31,13 +30,13 @@ class MarkdownExtension extends ClideExtension {
 
   @override
   Future<void> activate(ClideExtensionContext ctx) async {
+    // Ensure the retained nav exists so it records selections + emits
+    // loads whether or not the viewer is mounted (T-196). This handler
+    // only reveals the tab; the nav owns load + history.
+    ctx.readerNav.navFor(id, dataKey: 'path');
     _sub = ctx.messages.subscribe(publisher: id, channel: 'selection').listen((msg) {
-      final path = msg.data['path'] as String?;
-      if (path == null) return;
+      if (msg.data['path'] is! String) return;
       ctx.panels.activateTab(Slots.contextPanel, 'markdown.viewer');
-      WidgetsBinding.instance.addPostFrameCallback((_) {
-        ctx.messages.publish(id, 'load', {'path': path});
-      });
     });
   }