fix: backup existing config files before overwriting (#119)

Before writing config.toml (Codex) or opencode.json (OpenCode), the CLI
attempts to create a timestamped backup of any existing config file.
This prevents accidental data loss when users have customized configs.

Backup is best-effort - if it fails (e.g., unusual permissions), the
install continues without blocking.

Backup files are named: config.toml.bak.2026-01-23T21-16-40-065Z

src/targets/codex.ts

@@ -1,5 +1,5 @@
 import path from "path"
-import { copyDir, ensureDir, writeText } from "../utils/files"
+import { backupFile, copyDir, ensureDir, writeText } from "../utils/files"
 import type { CodexBundle } from "../types/codex"
 import type { ClaudeMcpServer } from "../types/claude"
 
@@ -30,7 +30,12 @@ export async function writeCodexBundle(outputRoot: string, bundle: CodexBundle):
 
   const config = renderCodexConfig(bundle.mcpServers)
   if (config) {
-    await writeText(path.join(codexRoot, "config.toml"), config)
+    const configPath = path.join(codexRoot, "config.toml")
+    const backupPath = await backupFile(configPath)
+    if (backupPath) {
+      console.log(`Backed up existing config to ${backupPath}`)
+    }
+    await writeText(configPath, config)
   }
 }
 

src/targets/opencode.ts

@@ -1,10 +1,15 @@
 import path from "path"
-import { copyDir, ensureDir, writeJson, writeText } from "../utils/files"
+import { backupFile, copyDir, ensureDir, writeJson, writeText } from "../utils/files"
 import type { OpenCodeBundle } from "../types/opencode"
 
 export async function writeOpenCodeBundle(outputRoot: string, bundle: OpenCodeBundle): Promise<void> {
   const paths = resolveOpenCodePaths(outputRoot)
   await ensureDir(paths.root)
+
+  const backupPath = await backupFile(paths.configPath)
+  if (backupPath) {
+    console.log(`Backed up existing config to ${backupPath}`)
+  }
   await writeJson(paths.configPath, bundle.config)
 
   const agentsDir = paths.agentsDir

src/utils/files.ts

@@ -1,6 +1,19 @@
 import { promises as fs } from "fs"
 import path from "path"
 
+export async function backupFile(filePath: string): Promise<string | null> {
+  if (!(await pathExists(filePath))) return null
+
+  try {
+    const timestamp = new Date().toISOString().replace(/[:.]/g, "-")
+    const backupPath = `${filePath}.bak.${timestamp}`
+    await fs.copyFile(filePath, backupPath)
+    return backupPath
+  } catch {
+    return null
+  }
+}
+
 export async function pathExists(filePath: string): Promise<boolean> {
   try {
     await fs.access(filePath)