feat(powershell): make help codigest show native PowerShell help without overriding help #5

Problem

codigest --help works, but PowerShell help codigest resolves to Get-Help codigest and currently says no help topic exists. PowerShell doesn't invoke external executables' --help flag.

Proposal

• Provide an opt-in PowerShell profile integration exposing comment-based help via a thin forwarder function named codigest (with usage/examples and link to full codigest --help).
• Ensure help codigest and Get-Help codigest work in PowerShell 7 and Windows PowerShell 5.1 after installation.
• Forward all arguments and preserve exit code / pipelines for ordinary codigest ... usage, without overriding PowerShell's global help function or changing unrelated commands.
• Idempotent install and clean uninstall; do not implicitly enable terminal autocapture as a prerequisite.
• Test installer idempotency and leave existing user profile content intact. Document how to enable it.

Acceptance

After opt-in setup and opening a new shell, help codigest prints useful synopsis, commands and examples; codigest --help remains fully functional.