dynamic context panel tabs — spawn on demand, no empty placeholders

Context panel tabs are no longer static. Extensions spawn them
dynamically when relevant and despawn on deactivate:
- Tickets: detail tab spawns on builtin.tickets/selection
- Decisions: detail tab spawns on builtin.decisions/selection
- Backlinks: spawns when editor opens a file
- Markdown viewer: spawns when editor opens a .md file
- Graph: removed from static context (will be command-spawnable)

The icon rail only shows what's active — no empty tabs sitting
there waiting for content.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

lib/builtin/markdown/src/extension.dart

@@ -1,3 +1,5 @@
+import 'dart:async';
+
 import 'package:clide/builtin/markdown/src/markdown_viewer.dart';
 import 'package:clide/extension/extension.dart';
 import 'package:clide/kernel/kernel.dart';
@@ -8,20 +10,51 @@ class MarkdownExtension extends ClideExtension {
   @override
   String get title => 'Markdown';
   @override
-  String get version => '0.1.0';
+  String get version => '0.2.0';
   @override
   List<String> get dependsOn => const ['builtin.editor'];
 
+  ClideExtensionContext? _ctx;
+  StreamSubscription<DaemonEvent>? _editorSub;
+  bool _viewerSpawned = false;
+
   @override
-  List<ContributionPoint> get contributions => [
-        TabContribution(
-          id: 'markdown.viewer',
-          slot: Slots.contextPanel,
-          title: 'Viewer',
-          titleKey: 'tab.viewer',
-          i18nNamespace: id,
-          priority: -100,
-          build: (_) => const MarkdownViewer(),
-        ),
-      ];
+  List<ContributionPoint> get contributions => const [];
+
+  @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();
+        }
+      }
+    });
+  }
+
+  @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');
+  }
 }