spacectl: Add --wait / exit-on-complete for stack tasks

In agent workflows, models often use spacectl stack task --tail, which doesn’t exit and stalls agents.

Add a --wait (or similar) mode that returns when the task completes (with success/failure exit code) and document it as the recommended option for automation/agents.

Example: spacectl stack task --id <stack> --wait "terraform import ..." exits when done; --tail remains for interactive streaming.