feat: add project language detection and upgrade command for skills

[qjrm1430]

• Implemented _detect_project_language to identify project language from README.md.
• Added _detect_first_cast to retrieve the first cast from langgraph.json.
• Introduced _upgrade_skills to manage skill upgrades and backups.
• Created upgrade_command to facilitate upgrading skills in existing Act projects.
• Enhanced integration tests to verify skill replacement and backup functionality.

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