slab: AI-optimize MCP action set

Slab today is read-only — it can get, list, and search posts. An agent can find things but can't actually maintain the knowledge base. We'd like these improved and new tools added so an agent can create and manage content too, and so each tool is easy for an agent to reach for and get right.

A few things an agent can't do today, for example:
- Create or update a post
- Read the full content of a single post
- Organize posts into topics
- Manage who can see a post

**API reference:** https://studio.apollographql.com/public/Slab/variant/current/home (Slab's API is GraphQL, not REST)

**Add marker:**
When a component is AI-optimized, add this as to the file:
```
ai: "optimized"
```
Add it to each action file.