chore: Add AGENTS.md (#567)

* Also Add CLAUDE.md with a pointer to AGENTS.md as a temporary workaround for the lack of support of AGENTS.md in claude code

Author: ValerianRey

AGENTS.md

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+- Only generate docstrings for public functions or functions that contain more than 4 lines of code.
+- Use the Sphinx style for Python docstrings (e.g. :param my_param: Does something) and never
+  include a :returns: key.
+- The code you generate should always contain type hints in the function prototypes (including
+  return type hints of None), but type hints are not needed when initializing variables.
+- We use uv for everything (e.g. we do `uv run python ...` to run some python code, and
+  `uv run pytest tests/unit` to run unit tests). Please prefer `uv run python -c ...` over
+  `python3 -c ...`
+- After generating code, please run `uv run ty check`, `uv run ruff check` and `uv run ruff format`.
+  Fix any error.
+- After changing anything in `src` or in `tests/unit` or `tests/doc`, please identify the affected
+  test files in `tests/` and run them with e.g.
+  `uv run pytest tests/unit/aggregation/test_upgrad.py -W error`. Fix any error, either in the
+  changes you've made or by adapting the tests to the new specifications.

CLAUDE.md

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+@AGENTS.md