docs: add AGENTS.md with contribution rules for AI agents

[priya-sundaram-dev]

Describe your change:

Adds a model-neutral AGENTS.md at the repo root, as suggested by @cclauss in #15119, so AI coding agents (and their humans) have a single place that records the conventions that most often trip up automated PRs.

The headline rule is the one that keeps closing bot PRs: algorithms-keeper closes any PR whose "Describe your change" section has no checked box, so the file tells agents to tick the template before submitting. It also summarizes the ruff / type-hint / doctest / naming / placement requirements and points at CONTRIBUTING.md as the source of truth (it never overrides it).

I used the vendor-neutral AGENTS.md filename (the emerging cross-tool convention) rather than a tool-specific one so every agent picks it up.

• Documentation change?

Checklist:

• I have read CONTRIBUTING.md.
• This pull request is all my own work -- I have not plagiarized.
• I know that pull requests will not be merged if they fail the automated tests.
• All filenames are in all lowercase characters with no spaces or dashes.
• All functions and variable names follow Python naming conventions.

[cclauss]

Nice but… This project uses uv and has no requirements.txt.

Would it be better to suggest commands like:
uvx ruff …
uvx pre-commit …
uvx pytest

[priya-sundaram-dev]

Good catch — done in e0fbc95. Swapped the pip install -r requirements.txt block for uvx/uv run, and the lint/pre-commit lines now use uvx ruff … and uvx pre-commit …. Added a one-liner noting the project is uv-managed (pyproject.toml/uv.lock, no requirements.txt) so future agents don't reach for pip.