Document X-Lightdash-Project header for MCP

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Summary

The source PR adds a new X-Lightdash-Project HTTP header that MCP clients can send on requests to /api/v1/mcp to pin a specific Lightdash project for the request. When the header is set, list_projects and set_project are hidden from the tool list and project access is still enforced by the caller's permissions.

Docs changes

• Added a new subsection under Custom Integration (For Developers) in the Lightdash MCP reference explaining what the header does, when to use it, where to find a project UUID, and how the project tools are hidden when it's set.
• Added Cursor (.cursor/mcp.json) and curl examples showing how to send the header.
• Noted that invalid UUIDs are ignored and that permissions are always re-checked, so the header can't be used to escalate access.

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Project	Status	Preview	Updated (UTC)
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