Add `CURSOR_AGENT` detection

[pavkam]

Can we add detection for Cursor agent using CURSOR_AGENT env variable?

https://cursor.com/docs/agent/terminal#disable-heavy-prompts-for-agent-sessions

Is the feature request related to a problem

Not really a problem, but it's annoying when the agent is running tests and this keeps triggering Rider's Diff tool to be triggered over and over.

Describe the solution

As simple env. check for the CURSOR_AGENT variable as it's being done for other AI tools.

Describe alternatives considered

Adding manual check for CURSOR_AGENT in code in a module initiazer:

DiffEngine.DiffRunner.Disabled = Environment.GetEnvironmentVariable("CURSOR_AGENT") != null;

Additional context

Sorry, would have make a patch myself but not having enough time ATM.

Thanks!