docs: add AGENTS.md with core commands

[devin-ai-integration]

Summary

Adds an AGENTS.md file to the repo root listing the core commands AI coding agents (and humans) need when working with this codebase: build, format, lint, check, test, xtask vendored-artifact checks, and the regeneration commands for docs/HELP.md and openapi.json. Also includes brief notes on CI requirements and the strict clippy lint policy.

Review & Testing Checklist for Human

• Verify the listed commands match what you actually want agents running (e.g., confirm cargo xtask check and the generate-* commands are correct)
• Check if any other commands or conventions should be mentioned (e.g., integration tests, environment variables, branch naming)

Notes

Docs-only change — no code modifications.

Link to Devin session: https://app.devin.ai/sessions/ef25177750b64712a934f59ef2c6d520
Requested by: @sachiniyer

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Summary by cubic

Adds AGENTS.md with the core cargo and cargo xtask commands to build, lint, test, check vendored artifacts, and regenerate docs/HELP.md and openapi.json. Also documents CI checks and our strict Clippy policy.

^{Written for commit 0111d68. Summary will update on new commits. Review in cubic}

[devin-ai-integration]

Original prompt from Sachin

Can you help me seed a really simple agent guidance file just listing out the core commands you need to know when working with this repo

[devin-ai-integration]

🤖 Devin AI Engineer

I'll be helping with this pull request! Here's what you should know:

✅ I will automatically:

• Address comments on this PR. Add '(aside)' to your comment to have me ignore it.
• Look at CI failures and help fix them

Note: I can only respond to comments from users who have write access to this repository.

⚙️ Control Options:

• Disable automatic comment and CI monitoring

[devin-ai-integration]

✅ Devin Review: No Issues Found

Devin Review analyzed this PR and found no bugs or issues to report.

[cubic-dev-ai]

No issues found across 1 file

_{Re-trigger cubic}