fix(config): make plugin entry migration atomic with temp-file + rename

Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-openagent)

Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>

src/shared/migrate-legacy-plugin-entry.test.ts

@@ -1,4 +1,6 @@
-import { afterEach, beforeEach, describe, expect, it } from "bun:test"
+/// <reference path="../../bun-test.d.ts" />
+
+import { afterEach, beforeEach, describe, expect, it, mock } from "bun:test"
 import { mkdirSync, readFileSync, rmSync, writeFileSync } from "node:fs"
 import { tmpdir } from "node:os"
 import { join } from "node:path"
@@ -53,6 +55,43 @@ describe("migrateLegacyPluginEntry", () => {
     })
   })
 
+  describe("#given renaming the temp file fails after writing the migrated config", () => {
+    describe("#when migrating the config", () => {
+      it("#then keeps the original config untouched and writes the migrated content to a sibling temp file", async () => {
+        const configPath = join(testDir, "opencode.json")
+        const originalContent = JSON.stringify({ plugin: ["oh-my-opencode@latest"] }, null, 2)
+        const tempPath = `${configPath}.tmp`
+        writeFileSync(configPath, originalContent)
+
+        const fs = await import("node:fs")
+        const originalRenameSync = fs.renameSync
+
+        mock.module("node:fs", () => ({
+          ...fs,
+          renameSync: () => {
+            throw new Error("simulated rename failure")
+          },
+        }))
+
+        try {
+          const { migrateLegacyPluginEntry } = await importFreshMigrationModule()
+
+          const result = migrateLegacyPluginEntry(configPath)
+
+          expect(result).toBe(false)
+          expect(readFileSync(configPath, "utf-8")).toBe(originalContent)
+          expect(readFileSync(tempPath, "utf-8")).toContain("oh-my-openagent@latest")
+          expect(readFileSync(tempPath, "utf-8")).not.toContain("oh-my-opencode")
+        } finally {
+          mock.module("node:fs", () => ({
+            ...fs,
+            renameSync: originalRenameSync,
+          }))
+        }
+      })
+    })
+  })
+
   describe("#given opencode.json contains pinned oh-my-opencode version", () => {
     describe("#when migrating the config", () => {
       it("#then preserves the version pin", async () => {

src/shared/migrate-legacy-plugin-entry.ts

@@ -1,4 +1,4 @@
-import { existsSync, readFileSync, writeFileSync } from "node:fs"
+import { closeSync, existsSync, fsyncSync, openSync, readFileSync, renameSync, writeFileSync } from "node:fs"
 import { applyEdits, modify } from "jsonc-parser"
 
 import { parseJsoncSafe } from "./jsonc-parser"
@@ -66,7 +66,15 @@ export function migrateLegacyPluginEntry(configPath: string): boolean {
       : JSON.stringify({ ...(parseResult.data as OpenCodeConfig), plugin: updatedPluginEntries }, null, 2) + "\n"
     if (!updated || updated === content) return false
 
-    writeFileSync(configPath, updated, "utf-8")
+    const tempPath = `${configPath}.tmp`
+    writeFileSync(tempPath, updated, "utf-8")
+    const tempFileDescriptor = openSync(tempPath, "r")
+    try {
+      fsyncSync(tempFileDescriptor)
+    } finally {
+      closeSync(tempFileDescriptor)
+    }
+    renameSync(tempPath, configPath)
     log("[migrateLegacyPluginEntry] Auto-migrated opencode.json plugin entry", {
       configPath,
       from: LEGACY_PLUGIN_NAME,