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/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