feat: add --beta sandbox for Docker-based local agent sandboxing (#3127)

* feat: add --beta sandbox for Docker-based local agent sandboxing

When running agents locally, users can now opt into sandboxed execution
via `--beta sandbox` or the interactive picker. This runs the agent
inside a Docker container (using pre-built ghcr.io/openrouterteam images)
with memory and CPU limits, providing filesystem/network isolation.

- Docker auto-installed if missing (OrbStack on macOS, docker.io on Linux)
- Reuses existing makeDockerRunner() pattern from Hetzner/GCP
- Container auto-cleaned up on process exit
- OpenClaw security warning skipped in sandbox mode (already isolated)
- Interactive picker shows Direct vs Sandboxed when Docker available

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

* fix: rename local machine to local

Signed-off-by: Ahmed Abushagur <ahmed@abushagur.com>

* fix: remove memory limits and move sandbox to cloud picker

- Remove --memory=4g --cpus=2 from docker run (breaks small VMs and recursive spawns)
- Replace sandbox sub-prompt with a "Local Machine (Sandboxed)" option
  in the cloud picker itself, shown when --beta sandbox is active
- Docker availability check happens later in local/main.ts (ensureDocker),
  not in the picker — so the option always appears with --beta sandbox

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

* docs: add --beta sandbox to README

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

---------

Signed-off-by: Ahmed Abushagur <ahmed@abushagur.com>
Co-authored-by: spawn-bot <spawn-bot@openrouter.ai>
Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
Co-authored-by: Ahmed Abushagur <ahmed@abushagur.com>

packages/cli/package.json

@@ -1,6 +1,6 @@
 {
   "name": "@openrouter/spawn",
-  "version": "0.29.3",
+  "version": "0.29.4",
   "type": "module",
   "bin": {
     "spawn": "cli.js"