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/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)