wire builtin.editor to the active buffer + open files from the tree

EditorController mirrors the daemon's active buffer into the Flutter
UI. Hydrates on mount (editor.active → editor.read), subscribes to
editor.opened / active-changed / edited / saved / closed, and pushes
local edits back via editor.set-content. A one-shot echo-suppression
flag keeps the daemon's authoritative edit echo from clobbering the
caret the user just moved.

Text surface is EditableText directly — no TextField / Material
import, so D-007's "bare WidgetsApp" guardrail carries into the
editor tab. JetBrainsMono via the shared mono-family constants;
cursor + selection colours come from the theme tokens.

File-tree click now calls ipc.request('editor.open', {path}) instead
of the kernel command registry (which wouldn't have reached the
daemon). The opened file becomes active, the editor tab's controller
sees editor.active-changed, and the text content reconciles.

Linux release bundle still builds; 174 app tests + 74 core tests
stay green.

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

app/lib/builtin/editor/src/extension.dart

@@ -1,17 +1,30 @@
+import 'package:clide_app/builtin/editor/src/editor_view.dart';
 import 'package:clide_app/extension/extension.dart';
+import 'package:clide_app/kernel/kernel.dart';
 
-/// Tier-0 stub. Real implementation lands in a later tier; the extension
-/// is registered so the extensions-ui surface can list it as "installed,
-/// not yet implemented" and its id is reserved.
+/// Tier-2 editor pane. Contributes a single workspace tab that
+/// renders the daemon's active buffer. Multi-file tabs live in the
+/// follow-up plan; today the pane is one-at-a-time.
 class EditorExtension extends ClideExtension {
   @override
   String get id => 'builtin.editor';
   @override
   String get title => 'Editor';
   @override
-  String get version => '0.0.0-stub';
+  String get version => '0.1.0';
   @override
   List<String> get dependsOn => const [];
+
   @override
-  List<ContributionPoint> get contributions => const [];
+  List<ContributionPoint> get contributions => [
+        TabContribution(
+          id: 'editor.active',
+          slot: Slots.workspace,
+          title: 'Editor',
+          titleKey: 'tab.title',
+          i18nNamespace: id,
+          priority: 80, // between Claude (90) and welcome (-100)
+          build: (_) => const EditorView(),
+        ),
+      ];
 }