fix(background-agent): add circuit breaker to prevent subagent infinite loops

Adds a configurable maxToolCalls limit (default: 200) that automatically
cancels background tasks when they exceed the threshold. This prevents
runaway subagent loops from burning unlimited tokens, as reported in #2571
where a Gemini subagent ran 809 consecutive tool calls over 3.5 hours
costing ~$350.

The circuit breaker triggers in the existing tool call tracking path
(message.part.updated/delta events) and cancels the task with a clear
error message explaining what happened. The limit is configurable via
background_task.maxToolCalls in oh-my-opencode.jsonc.

Fixes #2571

src/config/schema/background-task.ts

@@ -11,6 +11,8 @@ export const BackgroundTaskConfigSchema = z.object({
   /** Timeout for tasks that never received any progress update, falling back to startedAt (default: 1800000 = 30 minutes, minimum: 60000 = 1 minute) */
   messageStalenessTimeoutMs: z.number().min(60000).optional(),
   syncPollTimeoutMs: z.number().min(60000).optional(),
+  /** Maximum tool calls per subagent task before circuit breaker triggers (default: 200, minimum: 10). Prevents runaway loops from burning unlimited tokens. */
+  maxToolCalls: z.number().int().min(10).optional(),
 })
 
 export type BackgroundTaskConfig = z.infer<typeof BackgroundTaskConfigSchema>