fix: don't trust a partial dev-commands.json cache as fully discovered

Fixed

• Command discovery (discover-commands.sh) no longer trusts a partial .coding-crew/dev-commands.json as fully discovered. It used to skip re-running discovery the moment the cache file existed at all, regardless of which of the six fields (test/lint/typecheck/install/env/credential_target) it actually contained. That let a partial write — e.g. ensure-deps.sh's own install_mode/docker_service fields landing first because an earlier sprint's model dispatch failed (expired credentials, a timeout) before ever writing the six discovery fields — permanently starve every later sprint of full discovery, since the file "already existed." The skip now requires all six fields to be present (any value, including JSON null, which still means "checked, confirmed no such command") before trusting the cache; a cache missing even one field falls through and re-runs discovery.