cli: colorize nx --help output using owo-colors (behind a --color flag)

[lntutor]

Summary

• add a global --color <auto|always|never> option for CLI help
• use owo-colors terminal detection and a conservative cyan/green/yellow help palette
• keep piped output plain by default and give NO_COLOR precedence over explicit color settings
• cover color policy, flag parsing, and ANSI/plain rendering with tests

Verification

• /Users/loi/.cargo/bin/cargo test -p nx-cli
• /Users/loi/.cargo/bin/cargo fmt --all -- --check
• /Users/loi/.cargo/bin/cargo clippy -p nx-cli --all-targets -- -D warnings
• git diff --check
• binary checks for piped auto, never, always, and NO_COLOR=1 modes

Closes #83

[lntutor]

Current head f12a73d88af24f7f422c90dea3876c53baebb231 is mergeable and the visible CI surface on this exact SHA is fully green: cross-platform Check, Clippy, and Test jobs on Ubuntu, macOS, and Windows; Format; Build WASM Examples; CLI Multi-process Smoke; Benchmark Comparator Tests; Ubuntu Profiling Artifacts; Benchmark Regression Gate; and the aggregate CI Success check all completed successfully. There are no review threads or review-body comments open on the current PR state, so if this colorized-help change still matches the intended direction, it looks ready for maintainer review/merge when convenient.

[GianIac]

Thanks for the PR, feature works and matches #83.

A few things before merge:

• PR has conflicts with main (likely from #89 touching main() / real_main()). Please rebase and make sure Cli::Completions is still intercepted before the Tokio runtime is built.

• requested_color() silently falls back to Auto on invalid values (--color=foobar), while clap will error out on the same input right after. Either accept only auto|always|never and let clap handle the rest, or drop the pre-parser and read the value from clap matches.

• Replace .expect(...) on Cli::from_arg_matches with ? — real_main returns Result, no need to panic here.

Nits (optional)

• stdout_supports_color() allocates a String just to search for \x1b. std::io::IsTerminal or supports_color::on(...) would be cleaner.

Once rebased and the two changes above are in, happy to merge.