Require tests for templates/ entries; exempt archive/ (+ archive workflow)

[Aydin-ab]

Formalizes the archive/ convention from the recent declutter, and the tested vs test-exempt split between templates/ and archive/.

Validator (ci/validate_build_yaml.py)

• templates/ entries require a test — active templates are tested. Omitting it fails with an actionable message ("…move it under archive/ instead; archive/ entries are test-exempt") instead of pydantic's generic "Field required".
• archive/ entries are test-exempt — dir / compute_config may point under archive/ (config co-located at archive/.../<name>/configs/), test optional; the templates/-naming conventions are relaxed there.

rayapp already treats test as optional (tests_path is omitempty), so an archive/ entry publishes via tmpl-publish with nothing for its test-template stage to run — the fast-iteration / urgent-fix path: ship a template update without waiting on the test pipeline by giving it a test-free archive/ entry.

Skill docs

• New workflows/archive-template.md — archive-vs-delete bar, the retire flow (remove/repoint the BUILD.yaml entry → git mv to archive/events/<year>-<event>/ or archive/reference/ with configs/+content/), and the publish-from-archive-without-tests flow.
• SKILL.md pointer + the tested/test-exempt invariant; schema note on test.

Testing

validate_build_yaml.py: current BUILD.yaml (55 entries) passes; a templates/ entry with no test is rejected with the archive message; an archive/ entry validates with or without a test.