Install agent skills on nitrostack-py init and upgrade - #18

Summary

• Clone https://github.com/nitrocloudofficial/skills-python-sdk.git after init and copy skills into the eight agent directories (.cursor/skills, .claude/skills, .copilot/skills, .codex/skills, .gemini/skills, .antigravity/skills, .opencode/skills, .agents/skills).
• Record skillsVersion in .nitrostack.json. nitrostack-py upgrade reinstalls when the remote package.json version is newer.
• Clone failures warn and do not abort init or PyPI upgrade. --force on init overwrites existing skill dirs.

Test plan

• pytest tests/test_cli_skills.py tests/test_cli.py
• nitrostack-py init /tmp/skills-smoke --template python-starter --skip-install writes eight skill trees and .nitrostack.json
• Confirm SKILL.md files mention Python McpApplicationFactory / @tool, not TypeScript @McpApp