unify Google Drive plugin

[paris-oai]

What changed

This consolidates the Google Docs, Google Sheets, and Google Slides plugin guidance into the existing Google Drive plugin so the plugins repo now presents one Google file plugin instead of four separate ones.

Specifically:

• adds a real google-drive top-level skill that acts as the single routing entrypoint
• moves the Docs, Sheets, and Slides skill content into google-drive/skills/google-drive/references/... for progressive disclosure
• removes the separate google-docs, google-sheets, and google-slides plugin manifests and marketplace entries
• keeps the old static assets under google-drive/assets/legacy/... so the content move is mostly file-preserving
• updates the README highlighted-plugin example from google-slides to google-drive

Validation

• git diff --check
• jq empty .agents/plugins/marketplace.json
• jq empty plugins/google-drive/.codex-plugin/plugin.json
• verified only plugins/google-drive remains as a Google file plugin with both .codex-plugin/plugin.json and .app.json