feat(auto-update-checker): implement background auto-update with configurable pinning

- Run update check in background after startup (non-blocking)
- Auto-update pinned versions in config file when newer version available
- Add auto_update config option to disable auto-updating
- Properly invalidate package cache after config update
- Scoped regex replacement to avoid editing outside plugin array

🤖 GENERATED WITH ASSISTANCE OF [OhMyOpenCode](https://github.com/code-yeongyu/oh-my-opencode)

src/config/schema.ts

@@ -122,6 +122,7 @@ export const OhMyOpenCodeConfigSchema = z.object({
   google_auth: z.boolean().optional(),
   sisyphus_agent: SisyphusAgentConfigSchema.optional(),
   experimental: ExperimentalConfigSchema.optional(),
+  auto_update: z.boolean().optional(),
 })
 
 export type OhMyOpenCodeConfig = z.infer<typeof OhMyOpenCodeConfigSchema>