yoi/work-items/closed/20260531-082646-document-env-var-policy/resolution.md

Documented the environment-variable policy and current supported surface.

Implementation:

• Added docs/environment.md.
• Stated the preferred design rule: avoid new environment variables when profile/manifest/config/CLI/typed secret references are better.
• Documented core path variables, runtime/socket variables, the Pod runtime command override, credential env references, external compatibility variables, and build/test/example-only variables.
• Clarified that normal runtime does not implicitly load .env files.
• Added cleanup direction for shared test env guards, keeping path resolution centralized, and moving toward typed secret references.
• Linked the new page from docs/nix.md and installed it in the Nix package docs.

Validation:

• ./tickets.sh doctor
• git diff --check