Position scripts as graph adapters

Summary

• Reframes Once around graph-aware repository automation and content-addressed actions instead of script-first positioning.
• Updates the graph, scripts, cache, homepage, README, and RFC docs to describe scripts as adapters into the graph/action model.
• Aligns CLI descriptions, generated reference pages, and the built-in script rule schema wording with the new model.

Testing

• mise exec -- cargo run -p once-cli -- reference --out docs/reference
• mise exec -- cargo fmt --all -- --check
• mise exec -- cargo test -p once-frontend
• mise exec -- cargo test -p once-cli reference
• cd docs && mise exec -- aube ci && mise exec -- aube run build
• git diff --check

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