define core frame vs shipped extension boundary (D-046)

Core frame builtins are infrastructure the shell can't function
without. Content extensions (editor, claude, canvas, etc.) are
bundled but architecturally removable — users swap the surface,
not the data.

Removes builtin.jira stub: Jira belongs as a third-party
extension, not a frame builtin. Git is a split component —
daemon-side process stays in frame, UI surfaces are extensions.

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

app/lib/builtin/jira/jira.dart

@@ -1 +0,0 @@
-export 'src/extension.dart';

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

@@ -1,17 +0,0 @@
-import 'package:clide_app/extension/extension.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.
-class JiraExtension extends ClideExtension {
-  @override
-  String get id => 'builtin.jira';
-  @override
-  String get title => 'Jira';
-  @override
-  String get version => '0.0.0-stub';
-  @override
-  List<String> get dependsOn => const [];
-  @override
-  List<ContributionPoint> get contributions => const [];
-}

app/lib/kernel/kernel.dart

@@ -3,7 +3,7 @@
 ///
 /// Admission rule: the kernel owns anything whose second concurrent user
 /// would create incoherent state or divergent UX. External-interfacing
-/// work generally belongs to extensions (git, pql, Linear, Jira);
+/// work generally belongs to extensions (git, pql, Linear);
 /// external-interfacing *singletons* (OS clipboard, tray, keychain)
 /// belong here.
 ///