docs: document developer token access levels and add MCP client configs

[ZLeventer]

Summary

• Add a table documenting the three developer token access levels (test account, Basic, Standard) with what each can do and how to obtain it
• Add a note explaining the common "test accounts only" error and how to resolve it
• Add guidance for MCP end-users on how to navigate the Basic access application
• Add Claude Code and Cursor/VS Code configuration examples alongside the existing Gemini examples
• Rename the "Configure Gemini" section to "Configure your MCP client" since the server is client-agnostic

Motivation

Addresses #50 — multiple users have reported confusion when their developer token only works with test accounts. The README currently says "get a Developer Token" without explaining that new tokens start at test-account-only access and that a separate application is needed for production data.

The MCP client config examples address the implicit Gemini-only framing, since this server works equally well with Claude Code, Cursor, and other MCP clients.

Test plan

• Verify all links resolve correctly
• Confirm table renders properly on GitHub

[Raibaz]

Thanks for putting this together!

Left a couple comments.

[Raibaz]

This is not correct: new developer tokens may be automatically upgraded to Explorer access, which is sufficient to use the MCP server.

Besides, I don't want to duplicate existing documentation here, so I think we should just link to the right page in the Google Ads API documentation.

[Raibaz]

I like the idea of having instructions for different MCP clients, but since the format of the mcpServers block is the same, can we synthesize this?

[ZLeventer]

Thanks for the review!

• Replaced the duplicated access-levels content with a short pointer to the official access levels documentation.
• Consolidated the per-client config blocks into a single "Other MCP clients" note that references the existing mcpServers example.

[Raibaz]

I'd also mention that developer tokens must have at least explorer access here.

[Raibaz]

You may want to resolve conflicts here :)

[ZLeventer]

Ah thank ya

[ZLeventer]

Resolved the conflicts by rebasing onto current main (incorporating the Cloud Run/Docker changes from #65). Single clean commit now. Thanks again for the review!