feat: add gamma integration (api key auth + mcp tools + docs)

[AggManik]

Closes #468

Adds a complete Gamma integration to CORE. Gamma is an AI-powered tool for creating presentations, documents, and websites. This PR wires it into CORE so users can sync their Gamma activity automatically and interact with Gamma content directly from their workspace.

MCP Tools added

Tool	Description
gamma_list_presentations	List presentations with pagination
gamma_get_presentation	Get a specific presentation by ID
gamma_create_presentation	Generate a new presentation from a text prompt
gamma_list_documents	List documents with pagination
gamma_get_document	Get a specific document by ID
gamma_list_websites	List published websites with pagination

docs/toolkit/gamma.mdx — New doc page

Full toolkit documentation for the Gamma integration covering:

• Step-by-step connection guide
• Action use cases (with example prompts)
• Proactive use cases (scheduled sync scenarios)
• MCP tools reference table
• Trigger events

docs/toolkit/overview.mdx + docs/docs.json — Navigation updated

Added Gamma to the Knowledge & Documentation section in both the overview
page and the sidebar nav so it's discoverable.