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.ts

@@ -14,7 +14,14 @@
  */
 
 import { execSync } from 'node:child_process';
-import { existsSync, mkdirSync, readFileSync, writeFileSync } from 'node:fs';
+import {
+  existsSync,
+  mkdirSync,
+  readFileSync,
+  writeFileSync,
+  readdirSync,
+  copyFileSync,
+} from 'node:fs';
 import { homedir } from 'node:os';
 import { dirname, join, resolve } from 'node:path';
 import { fileURLToPath } from 'node:url';
@@ -536,6 +543,47 @@ export function readRosterAgentNames(mosaicHome = join(homedir(), '.config', 'mo
   return names;
 }
 
+/**
+ * Refresh the ACTIVE systemd user units from the freshly re-seeded copies.
+ *
+ * The re-seed updates `~/.config/mosaic/systemd/user/*.service`, but the units
+ * systemd actually runs live at `~/.config/systemd/user/`. Without this copy,
+ * shipped unit fixes (e.g. the socket-env change) never take effect after
+ * `mosaic update` until `mosaic fleet install` is re-run. Best-effort + scoped:
+ * only refreshes when a fleet is already installed (the active dir already
+ * carries `mosaic-*` units), so non-fleet hosts are untouched.
+ */
+export function refreshActiveFleetUnits(
+  mosaicHome = join(homedir(), '.config', 'mosaic'),
+  env: NodeJS.ProcessEnv = process.env,
+): { refreshed: string[]; ok: boolean; reason?: string } {
+  const src = join(mosaicHome, 'systemd', 'user');
+  const configHome = env['XDG_CONFIG_HOME'] ?? join(homedir(), '.config');
+  const dest = join(configHome, 'systemd', 'user');
+  if (!existsSync(src)) return { refreshed: [], ok: true };
+  // Only refresh when a fleet is already installed (active dir has mosaic units).
+  const fleetInstalled =
+    existsSync(dest) &&
+    readdirSync(dest).some((f) => f.startsWith('mosaic-') && f.endsWith('.service'));
+  if (!fleetInstalled) return { refreshed: [], ok: true };
+  const units = readdirSync(src).filter((f) => f.startsWith('mosaic-') && f.endsWith('.service'));
+  const refreshed: string[] = [];
+  for (const unit of units) {
+    try {
+      copyFileSync(join(src, unit), join(dest, unit));
+      refreshed.push(unit);
+    } catch {
+      // best-effort per unit
+    }
+  }
+  try {
+    execSync('systemctl --user daemon-reload', { stdio: 'ignore', timeout: 15_000 });
+  } catch {
+    // non-systemd host or no session bus — non-fatal
+  }
+  return { refreshed, ok: true };
+}
+
 /** Build the per-agent systemd relaunch commands (drain+relaunch via restart). */
 export function buildRelaunchCommands(agentNames: string[]): string[][] {
   return agentNames.map((name) => [