Add PowerShell tab completion

[liurenfeng94-ops]

Closes #1.

Adds a deterministic PowerShell completion script for the soleil CLI.

What changed:

• Adds completions/soleil.ps1 with native PowerShell completions for documented commands and options.
• Completes known values for --mode, --language/--lang, and --suite.
• Documents load, profile install, and removal steps in the README.
• Adds focused tests that keep the completion lists covered and duplicate-free.
• Includes completions/ in the npm package files list.

Validation:

• npm ci
• npm run check

Note: this PR was prepared with AI coding-agent assistance and then validated locally with the commands above.