refactor: dedupe the jscpd clones; drop the baseline loading gate

- Extract the shared New Session action into WorkspacesService.startSession
  (sidebar button and workspace browser both delegate; recent-Workspace
  targeting and the no-workspace clear live in one place).
- Fold the chip-insertion transaction shared by insert-ref and paste-upgrade
  into one InputMachine helper.
- Share the fixture's session-not-found guard across the sessionId-addressed
  catalog routes.
- Drop the AppFrame baselines-ready loading gate (user ruling: the bare
  status line reads worse than the shell's own pending rendering); both
  column occupants mount from first paint.

packages/client/ui-layout/tests/app-frame.spec.tsx

@@ -160,11 +160,13 @@ describe('AppFrame', () => {
     expect(slotCalls.map((c) => c.key)).toContain('conversation')
   })
 
-  it('keeps the loading branch until both object-layer baselines are ready', () => {
+  it('renders both column occupants before baselines settle (no loading gate)', () => {
+    // User ruling: the bare loading status looked worse than the shell's own
+    // pending rendering — both occupants mount from first paint.
     baselinesReady.current = false
-    const { slotCalls, getByRole } = mountFrame()
-    expect(getByRole('status').textContent).toContain('Loading workspaces and sessions')
-    expect(slotCalls.map((c) => c.key)).not.toContain('conversation')
+    const { slotCalls } = mountFrame()
+    expect(slotCalls.map((c) => c.key)).toContain('conversation')
+    expect(slotCalls.map((c) => c.key)).toContain('details')
   })
 
   it('sidebar slot receives live concession output as owner props', () => {