docs: add AI coding tools policy

[Xe]

At some level it would be nice to be able to have the following AI coding policy from an ideological standpoint:

Anubis does not accept code made primarily with the use of agentic AI tools such as Claude Code, Gemini CLI, GitHub Copilot, Zed, OpenCode, or any other similar tools. Please do not use them when contributing to this repo.

However, I'd be in violation by doing this because I have knowingly committed minor bits of code to the Anubis repo that were generated by AI tools (mostly things for smoke tests).

As such, Anubis is taking more of a centrist approach with regards to AI coding tools: regardless of what tool you use to make contributions to Anubis, when you sign off your code, you are taking responsibility for what you commit. You are also expected to understand what you are changing, what the implications are, and all other relevant factors.

If you use AI coding tools for a majority of your committed work, you MUST disclose it with the Assisted-by footer. The Anubis maintainers will be using tooling that looks for these footers and will prioritize scrutiny and level of attention appropriately.

In order to ensure compliance with this policy, language has been placed in AGENTS.md and CLAUDE.md to entice AI coding tools to add these footers.

Checklist:

• Added a description of the changes to the [Unreleased] section of docs/docs/CHANGELOG.md
• Added test cases to the relevant parts of the codebase
• Ran integration tests npm run test:integration (unsupported on Windows, please use WSL)
• All of my commits have verified signatures