fix(agents): enforce English prompting for all subagents (#58)

- Add Language Rule (MANDATORY) section in OmO Delegation_Rules
- Clarify that subagent prompts must always be in English
- Update background-task tool documentation with English requirement
- Update call-omo-agent tool documentation with English language rule
- LLMs perform significantly better with English prompts
- Improves consistency and performance across all agent-to-subagent communication

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

src/tools/background-task/constants.ts

@@ -9,9 +9,11 @@ Use this for:
 
 Arguments:
 - description: Short task description (shown in status)
-- prompt: Full detailed prompt for the agent
+- prompt: Full detailed prompt for the agent (MUST be in English for optimal LLM performance)
 - agent: Agent type to use (any agent allowed)
 
+IMPORTANT: Always write prompts in English regardless of user's language. LLMs perform significantly better with English prompts.
+
 Returns immediately with task ID and session info. Use \`background_output\` to check progress or retrieve results.`
 
 export const BACKGROUND_OUTPUT_DESCRIPTION = `Get output from a background task.