[Agata-Proxima]

Changes

• Remove the obsolete instruction to download or create a separate tenant-signing-key file.
• Document that the Control Plane delivers the organization-scoped tenant verification key during signed HTTPS enrollment or identity refresh.
• Clarify that the Engine node installer supports Windows and Linux, while the supplied deployment-executor production service installer is Linux/systemd-specific.
• Add a README section explaining the difference between installing the Engine node and installing the customer-managed deployment executor.

Documentation-only change.