feat(config): persist config.patch updates atomically

src/config/persistence.ts

@@ -0,0 +1,26 @@
+import { copyFileSync, existsSync, mkdirSync, renameSync, writeFileSync } from 'fs';
+import { dirname } from 'path';
+import { stringify } from 'yaml';
+import type { Config } from './schema.js';
+
+/**
+ * Persist config atomically:
+ * 1) Backup existing file to <path>.bak
+ * 2) Write new YAML to temp file
+ * 3) Rename temp file into place
+ */
+export function persistConfig(configPath: string, config: Config): void {
+  const dir = dirname(configPath);
+  mkdirSync(dir, { recursive: true });
+
+  const yaml = stringify(config);
+  const tmpPath = `${configPath}.tmp-${process.pid}-${Date.now()}`;
+  const backupPath = `${configPath}.bak`;
+
+  if (existsSync(configPath)) {
+    copyFileSync(configPath, backupPath);
+  }
+
+  writeFileSync(tmpPath, yaml, 'utf-8');
+  renameSync(tmpPath, configPath);
+}