Wire envd's existing PTY process capabilities through the full stack:
hostagent proto (4 new RPCs: PtyAttach, PtySendInput, PtyResize, PtyKill),
envdclient, sandbox manager, and a new WebSocket endpoint at
GET /v1/sandboxes/{id}/pty with bidirectional JSON message protocol.
Sessions use tag-based identity for disconnect/reconnect support,
base64-encoded PTY data for binary safety, and a 120s inactivity timeout.
After restoring a VM from snapshot, envd had already completed its initial
MMDS poll, so the metadata files in /run/wrenn/ and env vars retained values
from the original sandbox. Call POST /init after WaitUntilReady on both
resume and create-from-template paths to trigger envd to re-read MMDS.
