feat(cli): colorize help output

[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.