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.ts

@@ -176,6 +176,7 @@ export async function executeFleetReconcile(
   assertLocalRosterOnly(request.roster);
   const validateRoster = request.deps.validateRoster ?? defaultValidateRoster(request);
   await validateRoster(request.roster);
+  assertLifecycleSocketAuthority(request);
   const plan = scopePlan(await observeFleet(request.roster, request.deps), request.agentName);
 
   if (request.command === 'status' || request.command === 'doctor') {
@@ -477,6 +478,23 @@ function assertVerificationSafe(plan: FleetReconcilePlan): void {
   }
 }
 
+function assertLifecycleSocketAuthority(request: FleetReconcileRequest): void {
+  const mayStart =
+    request.command === 'start' ||
+    request.command === 'restart' ||
+    ((request.command === 'apply' || request.command === 'reconcile') &&
+      request.roster.agents.some(
+        (agent: FleetRosterV2Agent): boolean =>
+          agent.lifecycle.enabled && agent.lifecycle.desiredState === 'running',
+      ));
+  if (mayStart && request.roster.tmux.socketName === '') {
+    throw new FleetReconcileError(
+      'lifecycle-precondition-failed',
+      'Default-server lifecycle start is unsupported by the fixed named-socket systemd units.',
+    );
+  }
+}
+
 function targetAgentsFor(roster: FleetRosterV2, agentName?: string): readonly FleetRosterV2Agent[] {
   if (agentName === undefined) return roster.agents;
   const agent = roster.agents.find(