feat(installer): add config backup utility for safe upgrades

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

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

src/cli/config-manager/backup-config.ts

@@ -0,0 +1,32 @@
+import { copyFileSync, existsSync, mkdirSync } from "node:fs"
+import { dirname } from "node:path"
+
+export interface BackupResult {
+  success: boolean
+  backupPath?: string
+  error?: string
+}
+
+export function backupConfigFile(configPath: string): BackupResult {
+  if (!existsSync(configPath)) {
+    return { success: true }
+  }
+
+  const timestamp = new Date().toISOString().replace(/[:.]/g, "-")
+  const backupPath = `${configPath}.backup-${timestamp}`
+
+  try {
+    const dir = dirname(backupPath)
+    if (!existsSync(dir)) {
+      mkdirSync(dir, { recursive: true })
+    }
+
+    copyFileSync(configPath, backupPath)
+    return { success: true, backupPath }
+  } catch (err) {
+    return {
+      success: false,
+      error: err instanceof Error ? err.message : "Failed to create backup",
+    }
+  }
+}