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();

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

@@ -10,7 +10,6 @@ import {
   writeFileSync,
   type Stats,
 } from 'node:fs';
-import { homedir } from 'node:os';
 import { basename, dirname, isAbsolute, join, relative, resolve, sep } from 'node:path';
 import type { Command } from 'commander';
 import {
@@ -19,6 +18,7 @@ import {
   type FleetHarnessContext,
   type RuntimeName,
 } from './launch.js';
+import { defaultFleetDataHome } from '../fleet/fleet-agent-scaffold.js';
 
 export const FLEET_AGENT_PROFILE_SCHEMA = 1;
 const PROFILE_KEYS = [
@@ -47,6 +47,7 @@ const CREDENTIAL_FILES: Record<RuntimeName, string> = {
 export type FleetLaunchErrorCode =
   | 'SCHEMA_TOO_NEW'
   | 'PROFILE_INVALID'
+  | 'AGENT_NOT_SCAFFOLDED'
   | 'COMPOSITION_FAILED'
   | 'FIRST_AUTH_REFUSAL';
 
@@ -513,6 +514,12 @@ export function resolveFleetLaunchComposition(
     throw new FleetLaunchError('PROFILE_INVALID', `invalid fleet agent name: ${name}`);
   }
   const agentDir = join(roots.userHome, 'fleet', 'agents', name);
+  if (lstatIfPresent(agentDir) === undefined) {
+    throw new FleetLaunchError(
+      'AGENT_NOT_SCAFFOLDED',
+      `no such fleet agent '${name}' — run: mosaic fleet agent new ${name}`,
+    );
+  }
   assertRealDirectory(agentDir, 'fleet agent directory');
   const profilePath = join(agentDir, 'profile.json');
   const profileInfo = lstatIfPresent(profilePath);
@@ -687,8 +694,7 @@ export function registerFleetLaunchCommand(
     .allowExcessArguments(true)
     .action((name: string, opts: { dryRun?: boolean }, command: Command): void => {
       try {
-        const userHome =
+        const userHome = deps.userHome ?? defaultFleetDataHome();
-          deps.userHome ?? process.env['MOSAIC_USER_HOME'] ?? join(homedir(), '.mosaic');
         const passthrough = command.args.slice(1);
         const plan = resolveFleetLaunchComposition(
           name,