Operator 2026-06-16. Replaces the static UPGRADE_BASE_VERSION + leaky single
compose.ccci.yml overlay model: dynamic base = last-green(warm canonical) ->
main fallback -> skip; optional minimal per-recipe previous/ folder for
base-only version repairs (ignored for head, version-guarded, removable when
stale). Validated on discourse PR #4 (official-image switch the current overlay
masks). regall then sweeps all recipes for regressions on sonnet.
