refactor: use consistent pino logging throughout migration and startup code

assistant/src/migrations/log.ts

@@ -1,12 +1,23 @@
+import pino from 'pino';
+import { logSerializers } from '../util/log-redact.js';
+
 /**
- * Stderr-only logger for migration code. Using the pino logger during
- * migration is unsafe because pino initialization calls ensureDataDir(),
- * which pre-creates workspace destination directories and causes migration
- * moves to no-op.
+ * Standalone pino instance for migration code. This must NOT use getLogger()
+ * because that triggers ensureDataDir(), which pre-creates workspace
+ * destination directories and causes migration moves to no-op.
+ *
+ * Writes to stderr only — no log files that might not exist yet.
  */
+const migrationLogger: pino.Logger = pino(
+  { name: 'migration', level: 'info', serializers: logSerializers },
+  pino.destination(2),
+);
+
 export function migrationLog(level: 'info' | 'warn' | 'debug', msg: string, data?: Record<string, unknown>): void {
-  if (level === 'debug') return; // suppress debug-level migration noise
-  const prefix = level === 'warn' ? 'WARN' : 'INFO';
-  const extra = data ? ' ' + JSON.stringify(data) : '';
-  process.stderr.write(`[migration] ${prefix}: ${msg}${extra}\n`);
+  if (level === 'debug') return;
+  if (data) {
+    migrationLogger[level](data, msg);
+  } else {
+    migrationLogger[level](msg);
+  }
 }