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/decisions/src/extension.dart

@@ -1,7 +1,10 @@
+import 'dart:async';
+
 import 'package:clide/builtin/decisions/src/decision_detail_view.dart';
 import 'package:clide/builtin/decisions/src/decisions_view.dart';
 import 'package:clide/extension/extension.dart';
 import 'package:clide/kernel/kernel.dart';
+import 'package:clide/kernel/src/events/message_bus.dart';
 
 class DecisionsExtension extends ClideExtension {
   @override
@@ -9,10 +12,14 @@ class DecisionsExtension extends ClideExtension {
   @override
   String get title => 'Decisions';
   @override
-  String get version => '0.2.0';
+  String get version => '0.3.0';
   @override
   List<String> get dependsOn => const [];
 
+  ClideExtensionContext? _ctx;
+  StreamSubscription<Message>? _selectionSub;
+  bool _detailSpawned = false;
+
   @override
   List<ContributionPoint> get contributions => [
         TabContribution(
@@ -24,12 +31,40 @@ class DecisionsExtension extends ClideExtension {
           priority: -20,
           build: (_) => const DecisionsView(),
         ),
-        TabContribution(
-          id: 'decisions.detail',
-          slot: Slots.contextPanel,
-          title: 'Decision',
-          priority: -50,
-          build: (_) => const DecisionDetailView(),
-        ),
       ];
+
+  @override
+  Future<void> activate(ClideExtensionContext ctx) async {
+    _ctx = ctx;
+    _selectionSub = ctx.messages.subscribe(publisher: id, channel: 'selection').listen(_onSelection);
+  }
+
+  @override
+  Future<void> deactivate() async {
+    _selectionSub?.cancel();
+    _despawnDetail();
+  }
+
+  void _onSelection(Message msg) {
+    final ctx = _ctx;
+    if (ctx == null) return;
+    if (!_detailSpawned) {
+      ctx.panels.contribute(TabContribution(
+        id: 'decisions.detail',
+        slot: Slots.contextPanel,
+        title: 'Decision',
+        priority: -50,
+        build: (_) => const DecisionDetailView(),
+      ));
+      _detailSpawned = true;
+    }
+    ctx.panels.activateTab(Slots.contextPanel, 'decisions.detail');
+  }
+
+  void _despawnDetail() {
+    if (_detailSpawned) {
+      _ctx?.panels.uncontribute('decisions.detail');
+      _detailSpawned = false;
+    }
+  }
 }