New docs/environments.md, nav 'Virtual environments' (before Deploy):
- the rule: never touch system Python (danger callout)
- project-based isolation as Local .venv / Makefile-driven / Docker tabs,
each with a runnable snippet; Docker ties back to the deploy standard
- local dev with pyenv: why, official install link, shell init with
annotated lines, everyday use, per-project .python-version
- shell quality-of-life extras (flake8 alias, .local/bin + npm-global PATH),
cross-ref'd to Standards and Workflow
workflow.md pyenv bullet now points at the new page; index.md gains a card.
Verified in-browser: tabs switch (Local/Makefile/Docker), annotations and
admonitions render; mkdocs build --strict clean (cross-ref anchors resolve).
Signed-off-by: disqualifier <dev@disqualifier.me>
