Fix macOS GPG CI coverage

[danielrobbins]

Summary

• use a short temporary root for the GPG E2E test so gpg-agent sockets do not exceed macOS AF_UNIX path limits
• add one macOS 3.13 job to the fast CI tier so Darwin-specific POSIX failures are caught on PRs instead of only after merge

Investigation

python-ci runs #24 and #26 failed only on master push full-sweep jobs. The PR/feature-branch runs were green because macOS was excluded from the fast tier. The failing jobs were all test-full-sweep (macos-latest, 3.9..3.13), which points to a macOS-specific pytest failure rather than the docs/parser changes themselves.

The new GPG E2E test creates a GNUPGHOME, and gpg-agent creates sockets under that directory. macOS has a short AF_UNIX socket path limit, while pytest's default temporary directory on GitHub-hosted macOS runners lives under a long /private/var/folders/... path. This patch gives the GPG E2E test a short temp root.

Verification

• PYTHONPATH=src python -m pytest tests/test_gpg_e2e.py -q
• PYTHONPATH=src python -m pytest tests -q
• python -m ruff check .
• python -m mypy
• pushed branch CI: https://github.com/danielrobbins/keychain/actions/runs/27316990210

The pushed branch CI passed, including the new test-fast (macos-latest, 3.13) job.