chore: require LLM usage disclosure in the pull request template

[Zethson]

Adds the LLM usage section from scverse/pertpy#1165 to the PR template, asking which LLM or coding agent was used and confirming every changed line was reviewed, understood and tested by the author.
It links the scverse AI policy and states that PRs skipping the section are closed without review.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 83.78%. Comparing base (55f77ed) to head (34032ff).
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #4407   +/-   ##
=======================================
  Coverage   83.78%   83.78%           
=======================================
  Files         134      134           
  Lines       12691    12691           
=======================================
  Hits        10633    10633           
  Misses       2058     2058           

Flag	Coverage Δ
hatch-test.low-vers	80.49% <ø> (ø)
hatch-test.pre	83.58% <ø> (ø)

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[Zethson]

I'll take the liberty of merging this was Ilan approved it for anndata. Let me know if it needs to be adapted, please.