Add IPR agreement workflow using centralized AdCP signatures

[bokelley]

Summary

• Adds IPR Policy agreement enforcement for all contributors using the contributor-assistant GitHub Action
• Signatures are stored centrally in adcontextprotocol/adcp repository, so contributors only need to sign once across all AdCP-related repos
• Contributors who have already signed in the AdCP repo will automatically pass the check

Test plan

• Open this PR and verify the IPR check runs
• If you haven't signed the IPR policy before, verify the bot comments asking you to sign
• If you have signed before, verify the check passes automatically

🤖 Generated with Claude Code

[github-actions]

IPR Policy Agreement Required

Thank you for your contribution! Before we can accept your pull request, you must agree to our Intellectual Property Rights Policy.

By making a Contribution, you agree that:

• You grant the Foundation a perpetual, irrevocable, worldwide, non-exclusive, royalty-free copyright license to your Contribution
• You grant a patent license under any Necessary Claims
• You represent that you own or have sufficient rights to grant these licenses

To agree, please comment below with the exact phrase:

I have read the IPR Policy

You can read the full IPR Policy here.

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I have read the IPR Policy

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_{You can retrigger this bot by commenting recheck in this Pull Request. Posted by the CLA Assistant Lite bot.}