cli: add nx completions <shell> subcommand (bash/zsh/fish/powershell)

[lntutor]

Closes #84

Adds nx completions <shell> for Bash, Zsh, Fish, PowerShell, and Elvish.

Completion generation is handled immediately after CLI parsing, before Tokio or the Numax runtime is initialized. The generated script is written only to stdout.

Tests cover:

• non-empty output for every supported shell
• missing and invalid shell diagnostics
• no files created in an empty working directory

[lntutor]

Current head 3847e3e2685214224a5a1c4b9d5308f627b6db45 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 shell-completion feature still matches the intended direction, it looks ready for maintainer review/merge when convenient.

[GianIac]

TY !