Run source snippets in isolated template projects

[timzhong1024]

Summary

• accept an optional source file and snippet arguments after --
• copy file contents into a fresh isolated language template instead of executing in the source project
• pass arguments through Python, Node, Rust, Go, and .NET backends
• document the isolation boundary in both READMEs and the bundled agent skill

Semantics

run-code TOOLCHAIN FILE -- ARG... reads only FILE. It does not run in the file's existing project, discover that project's dependencies, or copy sibling files. Dependencies remain explicit through --package.

Verification

• cargo fmt --check
• cargo clippy --locked --all-targets -- -D warnings
• cargo test --locked (26 tests)
• cargo package --locked
• npm wrapper syntax and dry-run package validation
• real file execution with argument passthrough on Python, Node, Rust, Go, and .NET; every snippet confirmed the source project's README.md was not visible from its execution directory
• Node file execution with zod@4 in normal output mode