Refactor skill reminder hot reload context handling

[F16shen]

Summary

• move the skill reminder out of persisted conversation history and inject it as runtime context when assembling each request
• reload the skill snapshot at safe points so the current reminder stays aligned with hot reloaded skills
• include the current skill count in the reminder and update regression coverage for hot reload behavior

Testing

• /home/lixin/workspace/aish/.venv/bin/python -m pytest tests/test_skill_hot_reload.py tests/test_context_manager.py tests/test_ask_user_tool.py tests/test_provider_registry.py

[github-actions]

Thanks for the pull request. A maintainer will review it when available.

Please keep the PR focused, explain the why in the description, and make sure local checks pass before requesting review.

Contribution guide: https://github.com/AI-Shell-Team/aish/blob/main/CONTRIBUTING.md

[github-actions]

This pull request description looks incomplete. Please update the missing sections below before review.

Missing items:

• User-visible Changes
• Compatibility
• Change Type
• Scope