Add postprocessing to CI test suite

[tomchor]

Summary

• Adds Python + pynanigans setup to the CI workflow
• New postprocessing/00_postproc_ci.py driver runs 01_create_aaaa.py and 02_create_xyza.py against the ci_test simulation output
• Verifies aaaa.ci_test.nc and xyza.ci_test.nc are produced in addition to the existing simulation checks
• Fixes a minor simname construction bug in 01_create_aaaa.py and 02_create_xyza.py: an empty parameter suffix no longer produces a trailing underscore (e.g. "ci_test_" → "ci_test")

Test plan

• Trigger with /run-tests on this PR and confirm all verify checks pass
• Confirm aaaa.ci_test.nc and xyza.ci_test.nc are present in the verify step output

🤖 Generated with Claude Code

[tomchor]

/run-tests