Standardize on Python 3.13.x only

[ckrough]

Summary

Standardizes the project to require Python 3.13.x only (no support for Python 3.14+).

Changes

• pyproject.toml: Changed requires-python from >=3.13 to >=3.13,<3.14
• uv.lock: Regenerated with correct Python constraint (==3.13.*)
• Documentation: Updated README, CONTRIBUTING, and CLAUDE.md to reference Python 3.13.x
• CI/CD: Updated increments.md to reflect Python 3.13-only testing plan

Rationale

This ensures:

• Consistent behavior across all Python 3.13.x patch versions
• Simplified testing (single minor version)
• Clear expectations for users and contributors
• Alignment between package metadata and lockfile

Testing

• Version specifier follows best practices (>=3.13,<3.14)
• Lockfile regenerated successfully
• All documentation consistent
• GitHub Actions already configured for Python 3.13

Review Notes

All changes have been reviewed by the code-reviewer agent and address:

• Critical: Corrected version specifier format
• Major: Lockfile consistency with pyproject.toml
• Minor: Clarified patch version support in docs