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/call-omo-agent/constants.ts

@@ -21,4 +21,5 @@ Usage notes:
 3. Each agent invocation is stateless unless you provide a session_id
 4. Your prompt should contain a highly detailed task description for the agent to perform autonomously
 5. Clearly tell the agent whether you expect it to write code or just to do research
-6. For long-running research tasks, use run_in_background=true to avoid blocking`
+6. For long-running research tasks, use run_in_background=true to avoid blocking
+7. **IMPORTANT**: Always write prompts in English regardless of user's language. LLMs perform significantly better with English prompts.`