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});
-      });
     });
   }
 

lib/builtin/markdown/src/markdown_viewer.dart

@@ -12,30 +12,43 @@ class MarkdownViewer extends StatefulWidget {
   State<MarkdownViewer> createState() => _MarkdownViewerState();
 }
 
-class _MarkdownViewerState extends State<MarkdownViewer> with ReaderHistoryMixin<String, MarkdownViewer> {
+class _MarkdownViewerState extends State<MarkdownViewer> {
   String? _path;
   String? _content;
   String? _error;
   StreamSubscription<Message>? _selectionSub;
+  ReaderNav? _nav;
 
   @override
   void didChangeDependencies() {
     super.didChangeDependencies();
     if (_selectionSub != null) return;
     final kernel = ClideKernel.of(context);
+    // Back/forward history is the retained right-pane nav (T-196); it
+    // records selections and re-emits them on the 'load' channel.
+    _nav = kernel.readerNav.navFor('builtin.markdown', dataKey: 'path')..addListener(_onNavChanged);
     _selectionSub = kernel.messages.subscribe(publisher: 'builtin.markdown', channel: 'load').listen((msg) {
       final path = msg.data['path'] as String?;
       if (path != null) _loadFile(path);
     });
+    // Grab the latest entry the nav already holds (a selection that
+    // revealed this tab before we subscribed).
+    final current = _nav!.current;
+    if (current != null) _loadFile(current);
+  }
+
+  void _onNavChanged() {
+    if (mounted) setState(() {}); // refresh action-bar button state
   }
 
   @override
   void dispose() {
     _selectionSub?.cancel();
+    _nav?.removeListener(_onNavChanged);
     super.dispose();
   }
 
-  /// Load [path] and push it onto the history stack (external navigation).
+  /// Fetch + display [path]. History lives in [ReaderNav]; never pushes.
   Future<void> _loadFile(String path) async {
     final kernel = ClideKernel.of(context);
     final resp = await kernel.ipc.request('files.read', args: {'path': path});
@@ -47,50 +60,15 @@ class _MarkdownViewerState extends State<MarkdownViewer> with ReaderHistoryMixin
         _content = resp.data['content'] as String? ?? '';
         _error = null;
       });
-      historyPush(path);
     } else {
       setState(() => _error = resp.error?.message);
     }
   }
 
-  /// Load [path] WITHOUT pushing onto the history stack (back/forward nav).
-  Future<void> _loadFileInPlace(String path) async {
-    final kernel = ClideKernel.of(context);
-    final resp = await kernel.ipc.request('files.read', args: {'path': path});
-    if (!mounted) return;
-    if (resp.ok) {
-      kernel.messages.publish('builtin.markdown', 'focus', {'path': path});
-      setState(() {
-        _path = path;
-        _content = resp.data['content'] as String? ?? '';
-        _error = null;
-      });
-    } else {
-      setState(() => _error = resp.error?.message);
-    }
-  }
-
-  void _onBack() {
-    final entry = historyBack();
-    if (entry != null) _loadFileInPlace(entry);
-  }
-
-  void _onForward() {
-    final entry = historyForward();
-    if (entry != null) _loadFileInPlace(entry);
-  }
-
-  void _onPin() {
-    pinCurrent();
-  }
-
-  void _onJumpToPin() {
-    final entry = jumpToPin();
-    if (entry != null) {
-      // Navigate directly without re-pushing (pin jump is a quick-return).
-      _loadFileInPlace(entry);
-    }
-  }
+  void _onBack() => _nav?.back();
+  void _onForward() => _nav?.forward();
+  void _onPin() => _nav?.pin();
+  void _onJumpToPin() => _nav?.jumpToPin();
 
   void _onEdit() {
     final p = _path;
@@ -127,13 +105,13 @@ class _MarkdownViewerState extends State<MarkdownViewer> with ReaderHistoryMixin
       subtitle: '${_content!.split('\n').length} lines',
       trailing: [
         ReaderActionBar(
-          canGoBack: canGoBack,
-          canGoForward: canGoForward,
-          hasPinned: hasPinned,
-          onBack: canGoBack ? _onBack : null,
-          onForward: canGoForward ? _onForward : null,
+          canGoBack: _nav?.canGoBack ?? false,
+          canGoForward: _nav?.canGoForward ?? false,
+          hasPinned: _nav?.hasPinned ?? false,
+          onBack: (_nav?.canGoBack ?? false) ? _onBack : null,
+          onForward: (_nav?.canGoForward ?? false) ? _onForward : null,
           onPin: _path != null ? _onPin : null,
-          onJumpToPin: hasPinned ? _onJumpToPin : null,
+          onJumpToPin: (_nav?.hasPinned ?? false) ? _onJumpToPin : null,
           onEdit: _path != null ? _onEdit : null,
         ),
       ],