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)

Author: code-yeongyu

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>

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.

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.`