fix: present extension opt-in prompts in user's conversation language

[masamaru0513]

Summary

Extension opt-in prompts (*.opt-in.md) were hardcoded in English, causing non-English users to see questions in English regardless of their conversation language.

Closes #176

Changes

Added a single instruction line at the top of each opt-in prompt block directing the AI to present the question in the user's conversation language.

User experience

Before: Extension opt-in questions (Security Baseline, Property-Based Testing) always appeared in English even when conversing in Japanese.

After: Questions are presented in the user's conversation language (e.g., Japanese when conversing in Japanese).

Checklist

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• I have reviewed the contributing guidelines
• I have performed a self-review of this change
• Changes have been tested
• Changes are documented

Test Plan

1. Set up AI-DLC with extensions in Kiro CLI
2. Start with Using AI-DLC, 簡単なTodoアプリを作りたい
3. Answer initial questions in Japanese
4. Verify extension opt-in questions appear in Japanese

Acknowledgment

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