Modernize development environment with Nix Flakes

[Aurelian-Shuttleworth]

What was done

• Introduced Nix Flakes (flake.nix) and .envrc to provide a fully reproducible, hermetic development environment.
• Pinned all Python dependencies (including babel, truststore) via flake.lock.
• Updated tests/conftest.py to use dynamic dates (datetime.now()) to fix test rot.

Why

• Reproducibility: Eliminates "works on my machine" issues.
• Reliability: Ensures pytest runs in an identical environment locally and in CI.

Verification Steps

Automated Tests

Ran the full legacy test suite in the new hermetic environment:

$ nix develop --command pytest
tests/test_auth.py .                                                     [ 14%]
tests/test_conflicts.py ...                                              [ 19%]
tests/test_gcalcli.py ............................                       [ 63%]
tests/test_input_validation.py .......                                   [ 74%]
tests/test_printer.py ..........                                         [ 90%]
tests/test_utils.py ......                                               [100%]
======================== 63 passed, 1 warning in 1.25s =========================

[kody-ai]

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