feat: add IPR agreement workflow using centralized AdCP signatures

[bokelley]

Summary

• Adds .github/workflows/ipr-agreement.yml to enforce IPR policy agreement on pull requests
• Uses contributor-assistant/github-action to check and store signatures
• Signatures stored centrally in adcontextprotocol/adcp repo so contributors sign once across all org repos
• Bots and automated accounts exempted via allowlist

Test plan

• Open a test PR and verify the workflow triggers and posts the IPR agreement comment
• Comment I have read the IPR Policy and verify the workflow marks the PR as signed
• Verify bot accounts (dependabot, renovate) are not required to sign

Closes #38

🤖 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.}