docs(readme): say up front that TorroMail is an MCP server set up in an app

[mahype]

Summary

Follow-up to #46, which was merged before this commit landed.

• Title: TorroMail — MCP server for your email, so the first line says what the software is.
• Replaces the slogan under the badges with the differentiator: set up in a native macOS app (or the terminal UI on Linux and Windows) instead of JSON config files, per-account permissions, approval before anything is sent, moved or deleted, and everything local.

Test plan

• README header renders on GitHub
• Documentation-only; consistent with the product boundary in AGENTS.md

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