Remove TOML script targets

Summary

• Remove TOML-backed script target parsing so once.toml is reserved for workspace configuration.
• Switch script annotations to the capitalized # Once marker across parser tests, shellspecs, docs, and website examples.
• Update CI and local guidance to use once exec instead of the removed once run check manifest target.

Testing

• mise exec -- cargo fmt --all -- --check
• mise exec -- cargo test --workspace
• mise exec -- cargo test -p once-cli commands::run::action::script::tests::uncached_script_digest_is_stable_for_the_same_nonce
• mise exec -- cargo test -p once-frontend script::tests::parses_bash_script_annotations

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