feat: add uv run support

[alexanderkreidich]

Summary

• add a dedicated rtk uv command that preserves uv run environment semantics while filtering output down to relevant failures
• add discover/rewrite support so uv run ... rewrites to rtk uv run ...
• document the new command in the README, Python command notes, and hook initialization help

Testing

• PATH=$HOME/.rustup/toolchains/stable-x86_64-apple-darwin/bin:$PATH cargo test uv_run
• PATH=$HOME/.rustup/toolchains/stable-x86_64-apple-darwin/bin:$PATH cargo test test_init_mentions_all_top_level_commands
• PATH=$HOME/.rustup/toolchains/stable-x86_64-apple-darwin/bin:$PATH cargo run --quiet -- hook check -- uv run pytest
• ./target/debug/rtk uv run python -c 'print("hello")'
• ./target/debug/rtk uv run python -c 'raise RuntimeError("kaboom")'

Closes #1379

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