Add Google Slides plugin and skills

[Aismit]

Summary

Adds a new google-slides plugin to the curated plugins repo and wires in the three Google Slides Codex skills from openai/openai#752216:

• google-slides-visual-iteration
• google-slides-template-surgery
• google-slides-template-migration

This change includes:

• a new plugins/google-slides plugin wrapper
• Google Slides connector wiring in .app.json
• plugin manifest metadata and minimal plugin assets
• the three skill directories and their reference docs
• a new marketplace entry in .agents/plugins/marketplace.json

Why

We need these skills in the plugins repo so the Google Slides workflows are available through the curated plugin surface, not only as repo-local skills in openai/openai.

These skills cover three distinct Google Slides jobs that the base connector alone does not route clearly enough:

• visual cleanup of real decks with thumbnail-driven edit/verify loops
• structural template repair when local slide fixes are not converging
• migration of an existing deck onto a branded/company template

Without this plugin integration, the skills exist upstream but are not packaged for plugin discovery and install in this repo. Adding the plugin makes the Google Slides connector usable with purpose-built Codex guidance for iteration, surgery, and migration workflows.

Validation

• verified the imported skill files against openai/openai#752216
• ran git diff --check
• validated JSON with jq