Closes the Phase 3.2 UX loop on the CLI side — staging pushes have
been creating PreviewEnv rows since forge v0.14.0, and the
companion forge PR exposes /v1/preview-envs. This adds the cobra
subtree:

  grounds preview list           — table view of active preview envs
                                   (--include-deleted flag)
  grounds preview show <id>      — detail view (--json for raw output)
  grounds preview pin <id>       — opt env out of the TTL janitor
  grounds preview unpin <id>     — re-enable TTL sweep

internal/api/preview.go ships a thin client matching the new forge
routes (Push summary embedded so callers don't need a follow-up
GET /v1/pushes/<id>).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>