fix(install): preserve user fleet data on re-seed + refresh active units (CRITICAL) (#632)

Co-authored-by: Jason Woltje <jason@diversecanvas.com>
Co-committed-by: Jason Woltje <jason@diversecanvas.com>

packages/mosaic/src/runtime/update-checker.reseed.spec.ts

@@ -7,7 +7,9 @@ import {
   buildRelaunchCommands,
   readRosterAgentNames,
   runFrameworkReseed,
+  refreshActiveFleetUnits,
 } from './update-checker.js';
+import { existsSync, readFileSync } from 'node:fs';
 
 /**
  * F3-m3 / R13: `mosaic update` re-seeds the framework + (opt-in) relaunches
@@ -83,3 +85,41 @@ describe('runFrameworkReseed', () => {
     rmSync(missing, { recursive: true, force: true });
   });
 });
+
+describe('refreshActiveFleetUnits', () => {
+  let root: string;
+  let mosaicHome: string;
+  let configHome: string;
+
+  beforeEach(() => {
+    root = mkdtempSync(join(tmpdir(), 'mosaic-units-'));
+    mosaicHome = join(root, 'mosaic');
+    configHome = join(root, 'config');
+    mkdirSync(join(mosaicHome, 'systemd', 'user'), { recursive: true });
+    mkdirSync(join(configHome, 'systemd', 'user'), { recursive: true });
+    // Freshly re-seeded units (new content).
+    writeFileSync(join(mosaicHome, 'systemd', 'user', 'mosaic-agent@.service'), 'NEW\n');
+    writeFileSync(join(mosaicHome, 'systemd', 'user', 'mosaic-tmux-holder.service'), 'NEW\n');
+  });
+  afterEach(() => rmSync(root, { recursive: true, force: true }));
+
+  it('refreshes active units when a fleet is already installed', () => {
+    // Active dir already carries mosaic units (stale) → fleet is installed.
+    writeFileSync(join(configHome, 'systemd', 'user', 'mosaic-agent@.service'), 'OLD\n');
+    const res = refreshActiveFleetUnits(mosaicHome, {
+      XDG_CONFIG_HOME: configHome,
+    } as NodeJS.ProcessEnv);
+    expect(res.refreshed).toContain('mosaic-agent@.service');
+    expect(
+      readFileSync(join(configHome, 'systemd', 'user', 'mosaic-agent@.service'), 'utf-8'),
+    ).toBe('NEW\n');
+  });
+
+  it('is a no-op when no fleet is installed (active dir has no mosaic units)', () => {
+    const res = refreshActiveFleetUnits(mosaicHome, {
+      XDG_CONFIG_HOME: configHome,
+    } as NodeJS.ProcessEnv);
+    expect(res.refreshed).toEqual([]);
+    expect(existsSync(join(configHome, 'systemd', 'user', 'mosaic-agent@.service'))).toBe(false);
+  });
+});