Oldest-supported CLI acceptance gate and published 3-month compatibility promise

The Tuist server now guarantees backward compatibility for recent CLI releases. The server supports CLI versions from the last 3 months, and every production deployment is gated by an acceptance test that runs the oldest supported CLI against the canary environment. If a server change would break an older supported CLI, the deployment is blocked before it reaches production. A new Compatibility page in the CLI docs documents the 3-month support window and the deprecated behavior for older versions.