feat(cli): sandboxes command for Rexec BFF (#81)

* feat(cli): sandboxes command for Rexec BFF

Add pipeops sandboxes list/get/create/start/stop/restart/delete/session/usage
backed by pipeops-go-sdk v0.18.0 SandboxService. Also fix addon deploy
response mapping for current SDK Data shape.

* feat(cli): sandboxes exec for one-shot in-sandbox commands

Add pipeops sandboxes exec with --command/--workdir/--timeout, wiring
through ClientAPI to SandboxService.Exec.

* feat(cli): sandboxes files list/read for in-sandbox filesystem

pipeops sandboxes files list|read with --path for BFF-proxied file I/O.

* chore(cli): bump go-sdk to v0.18.1 and wire GitOps workspace

Pin sandboxes exec/files stack to the released SDK with live-test control-plane
alignments. Ensure CreateGitOps sets workspace_uuid from the selected workspace.

Author: 9trocode