fix(fleet): unify user data-home seam and add actionable unscaffolded-agent error

Integration reconciliation of T2/T3 seams on feat/wf-fleet-mvp:
- fleet launch now resolves the user root through defaultFleetDataHome()
  (MOSAIC_DATA_HOME), the same seam fleet agent new uses, instead of a
  divergent MOSAIC_USER_HOME variable.
- Launching an unscaffolded name raises AGENT_NOT_SCAFFOLDED with the
  actionable message pointing at 'mosaic fleet agent new <name>' (acceptance
  carried over from the T3 card after the roster-v2 reconciliation moved it
  onto the launch path).

Co-Authored-By: Claude Fable 5 <[email protected]>
Claude-Session: https://claude.ai/code/session_01Dtdjx4Gxude9fwyLezCrhh

packages/mosaic/src/commands/fleet-launch-command.spec.ts

@@ -122,6 +122,24 @@ describe('profile-selected overlay', () => {
   });
 });
 
+describe('unscaffolded agent names', () => {
+  it('points an unscaffolded name at mosaic fleet agent new', () => {
+    const fx = fixture();
+    try {
+      resolveFleetLaunchComposition('ghost', {
+        systemHome: fx.systemHome,
+        userHome: fx.userHome,
+      });
+      throw new Error('expected resolution to fail');
+    } catch (error: unknown) {
+      const launchError = error as FleetLaunchError;
+      expect(launchError.code).toBe('AGENT_NOT_SCAFFOLDED');
+      expect(launchError.message).toContain("no such fleet agent 'ghost'");
+      expect(launchError.message).toContain('mosaic fleet agent new ghost');
+    }
+  });
+});
+
 describe('A3 credential validation', () => {
   it('refuses a symlinked bundle credential file', () => {
     const fx = fixture();