Simplify docs direnv re-entry

[danielporterda]

Summary

• Simplifies docs environment re-entry so repo commands assume direnv exec and only run directly after the script re-enters with its marker.

Stack

This is PR 1/8 in the generated-reference automation split.

Base: main
Head: generated-reference-01-docs-env-direnv

Validation

• python3 -m pytest tests/test_docs_env.py; python3 -m py_compile scripts/docs_env.py; git diff --check

[danielporterda]

Closing this after splitting the stack: this docs_env cleanup is not required for the generated-reference automation series. The remaining stack now starts at #769 against main.