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/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.
 ///