Add universal agent type for skills management

[nicoalbanese]

Summary

Introduces a universal agent type to streamline skills management functionality, along with cleanup of third-party notices.

Key Changes

• Added universal agent type definition in src/agents.ts for managing skills across different agent implementations
• Updated type exports in src/types.ts to support the new agent type
• Removed obsolete ThirdPartyNoticeText.txt file

[vercel]

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