docs(skills): document first-time gws setup

[grasskin]

Summary

• document CLI and selective skill installation, including the required gws-shared dependency
• add the first-time gcloud and gws auth setup flow, including Cloud Terms and manual Desktop OAuth recovery steps
• document secure client-secret handling, explicit least-privilege Chat scopes, and the Chat-to-Slides scope set used in practice
• keep the generated gws-shared skill and its Rust generator synchronized, with assertions and a patch changeset

Related to #672. Documents a workaround for #822.

Validation

• quick_validate.py skills/gws-shared with PyYAML in an isolated uv environment
• git diff --check
• verified the generated shared-skill template matches skills/gws-shared/SKILL.md

cargo fmt and Rust tests were not run because Cargo is not installed in the local environment.

[changeset-bot]

🦋 Changeset detected

Latest commit: 9df60f0

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

Name	Type
@googleworkspace/cli	Patch

Not sure what this means? Click here to learn what changesets are.

Click here if you're a maintainer who wants to add another changeset to this PR

[google-cla]

Thanks for your pull request! It looks like this may be your first contribution to a Google open source project. Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

View this failed invocation of the CLA check for more information.

For the most up to date status, view the checks section at the bottom of the pull request.

[grasskin]

Closing because this documentation belongs in the Probie repository rather than upstream.