fix(migration): skip backup when config content is unchanged (fixes #3222)

src/shared/migration/config-migration.ts

@@ -11,6 +11,7 @@ export function migrateConfigFile(
   rawConfig: Record<string, unknown>
 ): boolean {
   const copy = structuredClone(rawConfig)
+  const originalSnapshot = JSON.stringify(rawConfig)
   let needsWrite = false
 
   // Load previously applied migrations from BOTH the legacy in-config
@@ -142,6 +143,22 @@ export function migrateConfigFile(
   }
 
   if (needsWrite) {
+    // Verify the config body actually changed before creating a backup and
+    // rewriting.  Several migration paths set needsWrite=true without
+    // altering the config body (e.g. stripping an already-absent
+    // _migrations field, or recording sidecar-only state).  Without this
+    // guard the plugin creates a new .bak.<timestamp> file on every
+    // startup even when the config is already up-to-date (#3222).
+    if (JSON.stringify(copy) === originalSnapshot) {
+      // Config body is identical after all migrations ran. Apply in-memory
+      // mutations (e.g. stripped _migrations) without touching the filesystem.
+      for (const key of Object.keys(rawConfig)) {
+        delete rawConfig[key]
+      }
+      Object.assign(rawConfig, copy)
+      return false
+    }
+
     const timestamp = new Date().toISOString().replace(/[:.]/g, "-")
     const backupPath = `${configPath}.bak.${timestamp}`
     let backupSucceeded = false