restructure pql sidebar: Search default, Markdown tab with viewer

Search (PQL DSL query) is now the default left tab; Markdown (filtered
to .md files) on the right.  Clicking a markdown file publishes on
the message bus; the markdown extension activates the context panel
viewer and re-publishes on a load channel after the frame so the
viewer is guaranteed to be mounted.  Viewer subscribes to load channel
and editor buffer events, publishes focus for sidebar highlighting.

Co-Authored-By: Claude <noreply@anthropic.com>

lib/builtin/markdown/src/extension.dart

@@ -3,6 +3,8 @@ import 'dart:async';
 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
@@ -10,51 +12,35 @@ class MarkdownExtension extends ClideExtension {
   @override
   String get title => 'Markdown';
   @override
-  String get version => '0.2.0';
+  String get version => '0.3.0';
   @override
-  List<String> get dependsOn => const ['builtin.editor'];
+  List<String> get dependsOn => const [];
 
-  ClideExtensionContext? _ctx;
-  StreamSubscription<DaemonEvent>? _editorSub;
-  bool _viewerSpawned = false;
+  StreamSubscription<Message>? _sub;
 
   @override
-  List<ContributionPoint> get contributions => const [];
+  List<ContributionPoint> get contributions => [
+        TabContribution(
+          id: 'markdown.viewer',
+          slot: Slots.contextPanel,
+          title: 'Markdown',
+          icon: PhosphorIcons.fileText,
+          build: (_) => const MarkdownViewer(),
+        ),
+      ];
 
   @override
   Future<void> activate(ClideExtensionContext ctx) async {
-    _ctx = ctx;
-    _editorSub = ctx.events.on<DaemonEvent>().listen((e) {
-      if (e.subsystem != 'editor') return;
-      if (e.kind == 'editor.active-changed' || e.kind == 'editor.opened') {
-        final path = e.data['path'] as String?;
-        if (path != null && path.endsWith('.md')) {
-          _spawnViewer();
-        }
-      }
+    _sub = ctx.messages.subscribe(publisher: id, channel: 'selection').listen((msg) {
+      final path = msg.data['path'] as String?;
+      if (path == null) return;
+      ctx.panels.activateTab(Slots.contextPanel, 'markdown.viewer');
+      WidgetsBinding.instance.addPostFrameCallback((_) {
+        ctx.messages.publish(id, 'load', {'path': path});
+      });
     });
   }
 
   @override
-  Future<void> deactivate() async {
-    _editorSub?.cancel();
-    if (_viewerSpawned) {
-      _ctx?.panels.uncontribute('markdown.viewer');
-      _viewerSpawned = false;
-    }
-  }
-
-  void _spawnViewer() {
-    final ctx = _ctx;
-    if (ctx == null || _viewerSpawned) return;
-    ctx.panels.contribute(TabContribution(
-      id: 'markdown.viewer',
-      slot: Slots.contextPanel,
-      title: 'Viewer',
-      priority: -100,
-      build: (_) => const MarkdownViewer(),
-    ));
-    _viewerSpawned = true;
-    ctx.panels.activateTab(Slots.contextPanel, 'markdown.viewer');
-  }
+  Future<void> deactivate() async => _sub?.cancel();
 }