ci: remove python3.13t builds

[cpcloud]

Closes #1177.

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Greptile Overview

Greptile Summary

This PR removes all Python 3.13t (free-threading) build configurations from the CI test matrix in ci/test-matrix.json. The change eliminates 6 test entries: 2 from Linux pull-request matrix (amd64 and arm64), 2 from Windows pull-request matrix (both amd64 with different LOCAL_CTK settings), and 2 from Windows nightly matrix. Python 3.14t free-threading builds are retained.

The rationale is that Python 3.13t had significant stability issues, particularly non-thread-safe ctypes which cuda-python's cuda_pathfinder package heavily relies on. Since Python 3.14t provides stable free-threading support, the team is skipping 3.13t entirely to avoid maintaining experimental support for an experimental interpreter. This pragmatic approach reduces maintenance burden while preserving free-threading capabilities through the more mature 3.14t implementation.

Potential Issues

None identified. The changes are purely CI configuration updates removing unstable test configurations. JSON formatting is correct and matrix ordering is preserved.

Confidence: 5/5 - This is a straightforward CI configuration change with clear motivation and no code impact.

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[cpcloud]

Since Python 3.14t provides stable free-threading support

This feels ... overly optimistic 😆

[cpcloud]

/ok to test

[mdboom]

LGTM!

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[leofang]

Q: We don't need 3.14t here? (TBH I don't remember why this if was needed.)