Add lib/hook/sql/README.md describing bundle layout, apply order, Nix API (self.lib.hectic.*), shell helper contract, and the steps for adding a new SQL file. Rewrite db-tool README's hectic section: drop stale PG_HECTIC_INHERITANCE / HECTIC_INHERITANCE_SQL env vars, add HECTIC_DOTENV_FILE, document the postgres-init / migrator init / database hydrate responsibility split.