chore: add AGENTS.md for coding assistants

[nicholasjameshall]

Description

Adds an AGENTS.md file with agent-specific instructions and a CLAUDE.md file which points to it.

Benefits:

1. Agent-specific context: the file points to agent-friendly resources which are otherwise difficult for it to discover. For example, https://ucp.dev/llms.txt, which contains a progressive-disclosure text-only representation of the schema documentation, and the pure text version of our CONTRIBUTING.md file. Right now, if users clone this repository ("ucp"), CONTRIBUTING.md is not copied there at all since it's in the .github repo. The agent doesn't find it.
2. High degree of discoverability: in testing with Claude, it successfully retrieves the CLAUDE.md file (as expected). Cursor and Copilot are likewise configured to look for an AGENTS.md file (amongst others). We can help to ensure that these coding agents follow the guidelines and have access to useful commands, and in turn make developers' lives a bit easier.
3. Up-to-date information: the CONTRIBUTING.md doesn't mention scripts which are actually being used to generate documentation (e.g. build_local.sh). We should probably also update the CONTRIBUTING.md, but there is arguably still value to having this information also locally in the AGENTS.md file since 1) it contains context which maybe isn't super useful for the LLM, and 2) if we simplify link to the CONTRIBUTING.md file the user has to constantly approve http requests to get the content at the beginning of each session.

This doesn't touch any of the governance / contributing docs but let me know if it should be reviewed as such.

Category (Required)

Please select one or more categories that apply to this change.

• Core Protocol: Changes to the base communication layer, global context, or breaking refactors. (Requires Technical Council approval)
• Governance/Contributing: Updates to GOVERNANCE.md, CONTRIBUTING.md, or CODEOWNERS. (Requires Governance Council approval)
• Capability: New schemas (Discovery, Cart, etc.) or extensions. (Requires Maintainer approval)
• Documentation: Updates to README, or documentations regarding schema or capabilities. (Requires Maintainer approval)
• Infrastructure: CI/CD, Linters, or build scripts. (Requires DevOps Maintainer approval)
• Maintenance: Version bumps, lockfile updates, or minor bug fixes. (Requires DevOps Maintainer approval)
• SDK: Language-specific SDK updates and releases. (Requires DevOps Maintainer approval)
• Samples / Conformance: Maintaining samples and the conformance suite. (Requires Maintainer approval)
• UCP Schema: Changes to the ucp-schema tool (resolver, linter, validator). (Requires Maintainer approval)
• Community Health (.github): Updates to templates, workflows, or org-level configs. (Requires DevOps Maintainer approval)

Related Issues

Checklist

• I have followed the Contributing Guide (including Conventional Commits title requirements and ! for breaking changes).
• I have updated the documentation (if applicable).
• My changes pass all local linting and formatting checks.
• I have added tests that prove my fix is effective or that my feature works.
• New and existing unit tests pass locally with my changes.
• (For Core/Capability) I have included/updated the relevant JSON schemas.
• I have regenerated Python Pydantic models by running generate_models.sh under python_sdk.

Screenshots / Logs (if applicable)