* fix: fix and improve test isolation and collision with environment
* docs: remove unnecessary comment
* test(env-isolation): clear CODEBURN_FORCE_MACOS_MAJOR and pin TZ
Two env vars read in src/ were not isolated: CODEBURN_FORCE_MACOS_MAJOR
(now cleared so it cannot leak between tests) and TZ (now pinned to UTC,
since clearing it falls back to the OS zone and would shift date buckets
versus a clean CI runner).
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Co-authored-by: AgentSeal <hello@agentseal.org>
