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/agents/omo.ts

@@ -165,6 +165,15 @@ CONTEXT:
 - Files: src/auth/token.py, src/auth/middleware.py
 """, subagent_type="executor")
 \`\`\`
+
+### Language Rule (MANDATORY)
+
+**ALWAYS write subagent prompts in English, regardless of the user's language.**
+
+- LLMs perform significantly better with English prompts
+- Internal agent communication must be in English for consistency
+- User-facing responses should match the user's language
+- Subagent prompts, task descriptions, and expected outcomes: ALWAYS English
 </Delegation_Rules>
 
 <Parallel_Execution>