feat(mosaic): unified first-run UX wizard -> gateway install -> verify (#418)

packages/mosaic/src/wizard.ts

@@ -1,3 +1,6 @@
+import { writeFileSync } from 'node:fs';
+import { join } from 'node:path';
+import { tmpdir } from 'node:os';
 import type { WizardPrompter } from './prompter/interface.js';
 import type { ConfigService } from './config/config-service.js';
 import type { WizardState } from './types.js';
@@ -11,6 +14,25 @@ import { runtimeSetupStage } from './stages/runtime-setup.js';
 import { skillsSelectStage } from './stages/skills-select.js';
 import { finalizeStage } from './stages/finalize.js';
 
+// ─── Transient install session state (CU-07-02) ───────────────────────────────
+
+const INSTALL_STATE_FILE = join(
+  process.env['XDG_RUNTIME_DIR'] ?? process.env['TMPDIR'] ?? tmpdir(),
+  'mosaic-install-state.json',
+);
+
+function writeInstallState(mosaicHome: string): void {
+  try {
+    const state = {
+      wizardCompletedAt: new Date().toISOString(),
+      mosaicHome,
+    };
+    writeFileSync(INSTALL_STATE_FILE, JSON.stringify(state, null, 2) + '\n', { mode: 0o600 });
+  } catch {
+    // Non-fatal — gateway install will just ask for home again
+  }
+}
+
 export interface WizardOptions {
   mosaicHome: string;
   sourceDir: string;
@@ -92,4 +114,8 @@ export async function runWizard(options: WizardOptions): Promise<void> {
 
   // Stage 9: Finalize
   await finalizeStage(prompter, state, configService);
+
+  // CU-07-02: Write transient session state so `mosaic gateway install` can
+  // pick up mosaicHome without re-prompting.
+  writeInstallState(state.mosaicHome);
 }