Add type hint style guidance to AGENTS.md

[drbenvincent]

Summary

• Adds a one-line style rule to the Type Checking section of AGENTS.md documenting the convention to use modern Python 3.10+ type hint syntax: X | None (not Optional[X]), lowercase dict/list/tuple (not Dict/List/Tuple from typing), and Literal for constrained string params.
• Motivated by review feedback from @juanitorduz on PR Transfer-Function ITS: Graded Interventions with Saturation & Adstock Transforms #548 where these conventions were applied across the codebase.

Test plan

• Documentation-only change, no code affected.

Made with Cursor

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.63%. Comparing base (34aa8ef) to head (4dffe08).
⚠️ Report is 3 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #739      +/-   ##
==========================================
+ Coverage   94.60%   94.63%   +0.02%     
==========================================
  Files          46       46              
  Lines        7602     7602              
  Branches      462      462              
==========================================
+ Hits         7192     7194       +2     
+ Misses        249      248       -1     
+ Partials      161      160       -1     

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[read-the-docs-community]

Documentation build overview

📚 causalpy | 🛠️ Build #31562143 | 📁 Comparing 4dffe08 against latest (34aa8ef)

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Show files changed (1 files in total): 📝 1 modified | ➕ 0 added | ➖ 0 deleted

File	Status
404.html	📝 modified