Script and CLI positioning updated for graph-aware automation

Once is repositioned around graph-aware repository automation rather than script-first caching. Documentation, the README, CLI help text, and generated reference pages now describe scripts as adapters that plug into the same target/action model. The once exec command is reframed as a low-level action surface for direct commands and script adapters, and the run command documentation now emphasizes resolving a target against the workspace graph and executing its capability through the action cache.