feat: add Vercel Sandbox backend

Adds Vercel Sandbox as a supported Hermes terminal backend alongside
existing providers (Local, Docker, Modal, SSH, Daytona, Singularity).

Uses the Vercel Python SDK to create/manage cloud microVMs, supports
snapshot-based filesystem persistence keyed by task_id, and integrates
with the existing BaseEnvironment shell contract and FileSyncManager
for credential/skill syncing.

Based on #17127 by @scotttrinh, cherry-picked onto current main.

cli.py

@@ -497,6 +497,7 @@ def load_cli_config() -> Dict[str, Any]:
         "singularity_image": "TERMINAL_SINGULARITY_IMAGE",
         "modal_image": "TERMINAL_MODAL_IMAGE",
         "daytona_image": "TERMINAL_DAYTONA_IMAGE",
+        "vercel_runtime": "TERMINAL_VERCEL_RUNTIME",
         # SSH config
         "ssh_host": "TERMINAL_SSH_HOST",
         "ssh_user": "TERMINAL_SSH_USER",