feat(fleet): add reviewed v1-to-v2 migration preview

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

packages/mosaic/src/fleet/fleet-reconciler.spec.ts

@@ -274,6 +274,53 @@ describe('fleet roster-owned reconciler', (): void => {
     ]);
   });
 
+  it.each(['start', 'apply'] as const)(
+    'fails closed before %s can start fixed named-socket services for a default-server roster',
+    async (command) => {
+      const calls: string[][] = [];
+      const defaultServerRoster: FleetRosterV2 = {
+        ...roster,
+        tmux: { ...roster.tmux, socketName: '' },
+        agents: [
+          {
+            ...roster.agents[0]!,
+            lifecycle: { enabled: true, desiredState: 'running' },
+          },
+        ],
+      };
+
+      await expect(
+        executeFleetReconcile({
+          roster: defaultServerRoster,
+          command,
+          expectedGeneration: 7,
+          deps: deps({
+            readRoster: async () => defaultServerRoster,
+            runner: async (executable, args) => {
+              calls.push([executable, ...args]);
+              if (executable === 'tmux' && args.includes('list-sessions')) {
+                return { stdout: '', stderr: '', exitCode: 1 };
+              }
+              return { stdout: '', stderr: '', exitCode: 0 };
+            },
+          }),
+        }),
+      ).rejects.toMatchObject({ code: 'lifecycle-precondition-failed' });
+      expect(calls).not.toContainEqual([
+        'systemctl',
+        '--user',
+        'start',
+        'mosaic-tmux-holder.service',
+      ]);
+      expect(calls).not.toContainEqual([
+        'systemctl',
+        '--user',
+        'start',
+        'mosaic-agent@coder0.service',
+      ]);
+    },
+  );
+
   it('starts only an explicitly running roster agent with exact systemd targets', async (): Promise<void> => {
     const calls: string[][] = [];
     const runningRoster: FleetRosterV2 = {