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.

tests/tools/test_command_guards.py

@@ -73,6 +73,10 @@ class TestContainerSkip:
         result = check_all_command_guards("rm -rf /", "daytona")
         assert result["approved"] is True
 
+    def test_vercel_sandbox_skips_both(self):
+        result = check_all_command_guards("rm -rf /", "vercel_sandbox")
+        assert result["approved"] is True
+
 
 # ---------------------------------------------------------------------------
 # tirith allow + safe command