Add a single-db mode to the dev database.yml helper

The dev helper wrote only the decomposed and single-db-ci-connection shapes. CI's plain single-db jobs run one connection with no ci entry, so a spec bisect against them needed a hand-built database.yml. The new mode sets DECOMPOSED_DB=false and CI_CONNECTION_DB=false on the clone's own setup_database_yml and keeps its statement_timeout. The self-test covers the variables, the exit code, the timeout line and the unknown-mode message.