CLI reports version incompatibility instead of connection error when server is unreachable

[tomkis]

Pre-requisities

• I am using the newest version of the platform

Describe the bug
When the CLI is configured to connect to a non-existent backend (agentstack-server not running or wrong URL), the error is surfaced as a version incompatibility rather than a clear connection failure. This is misleading — users may think they have a version mismatch when the server is simply not reachable.

To Reproduce

1. Configure CLI to point to a non-running agentstack-server
2. Run any CLI command that communicates with the backend
3. Observe the error message suggests version incompatibility

Expected behavior
The CLI should clearly indicate that the backend server is unreachable, e.g. "Cannot connect to Agent Stack server at . Is it running?"

Additional context
The version check in agentstack version already handles this partially — it shows "not running" when it can't fetch the server version. But other commands may catch the connection error and present it in a confusing way.