Cleanup strategy implementation

Question

Implement the four continuous cleanup rules:

1. Every file has a home before it is created. No "it will be moved later" — it won't be.
2. Move with git mv, never delete-and-recreate. History and blame survive a rename; they don't survive delete + new file.
3. Superseded documents are archived, not deleted, not left in place. Move to docs/archive/.
4. Ignored directories are emptied, not curated. scratch/ and dist/ are wiped when in the way. Anything that hurts to lose was in the wrong zone.

Pre-release checks:

git status --short          # Working tree clean?
git ls-files --others -i -c --exclude-standard   # Anything tracked that should be ignored?
git ls-files | grep -v '^\(src\|docs\|tests\|assets\|scripts\)/'   # What is loose in the root?

The third command should return only root files from §3.

Decision needed: Document and enforce these rules in contributing guidelines and CI.

Label: wayfinder:task
Part of #2517

Start by reading the existing contributing guidelines and CI configuration, then inspect the repository with the three listed git checks. Document the four cleanup rules, including docs/archive/, scratch/, and dist/, and add CI enforcement for the stated checks. Done means the guidelines contain the rules and CI verifies the intended repository layout.