feat(script): align script rules with annotated files

Summary

• make rule = "script" the canonical script model, with [target.script] and shared field names across inline scripts and annotated script files
• add script-aware execution support in fabrik run and fabrik exec, including direct shebang execution and autodetection for fabrik exec -- <runtime> <script>
• refresh the docs and website around script files and script rules, and remove older task and runtime-script documentation from the public path
• extend Rust and shellspec coverage for script parsing, lowering, caching, and direct execution flows

Testing

• mise exec -- cargo test -p fabrik-frontend --lib -- manifest script
• mise exec -- cargo test -p fabrik-cli --bin fabrik commands::run::action::script -- --nocapture
• mise exec -- cargo test -p fabrik-cli --bin fabrik commands::exec -- --nocapture
• mise exec -- cargo build --release -p fabrik-cli
• mise exec -- shellspec spec/run_spec.sh
• mise exec -- shellspec spec/exec_spec.sh
• npm --prefix docs run build

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