Agent mode must be aware of active tasks

[Tyriar]

I frequently see agent mode compile or re-run a watch command when it's actually already running in a task.

[meganrogge]

@Tyriar I think this is a duplicate of https://github.com/microsoft/vscode-copilot/issues/18734

[meganrogge]

we'll keep this open and close the other one

[meganrogge]

See https://github.com/microsoft/vscode-copilot/issues/18734#issuecomment-2994425841