Add code quality and linting guidelines to AGENTS.md

[drbenvincent]

Introduces a new section detailing code quality checks, including pre-commit usage, ruff commands for linting and formatting, and notes on linting rules and documentation exclusions.

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📚 Documentation preview 📚: https://causalpy--572.org.readthedocs.build/en/572/

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.95%. Comparing base (1fc193b) to head (5afc776).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #572   +/-   ##
=======================================
  Coverage   91.95%   91.95%           
=======================================
  Files          33       33           
  Lines        4776     4776           
=======================================
  Hits         4392     4392           
  Misses        384      384           

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