docs(skills): clarify selective installs

[Bortlesboat]

Summary

• add gws-shared to the selective install examples in the README
• update generated prerequisite text so selective installs point to gws-shared, while local checkout flows still mention gws generate-skills
• regenerate the affected skills/gws-* docs and add a changeset

Why

Issue #672 points out that service skills depend on gws-shared, but the current selective install examples omit it. The generated prerequisite text also only mentions gws generate-skills, which is right for local development but not for people installing a small subset from GitHub.

Verification

• cargo test -p google-workspace-cli prerequisite_mentions_selective_install
• cargo run -q -p google-workspace-cli -- generate-skills --output-dir skills

Addresses #672.

[changeset-bot]

🦋 Changeset detected

Latest commit: 08566e0

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

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