chore: remove simulation sandbox placeholder and references

Remove the empty sandboxes/simulation/ directory (contained only a
.gitkeep) and clean up all associated references in README.md:
- Remove table entry for sandboxes/simulation/
- Remove simulation from the --from flag example list
- Remove 'spin up physics simulations to master tasks' from intro

Author: drew

README.md

@@ -1,6 +1,6 @@
 # OpenShell Community
 
-[OpenShell](https://github.com/NVIDIA/OpenShell) is the runtime environment for autonomous agents -- the infrastructure where they live, work, and verify. It provides a programmable factory where agents can spin up physics simulations to master tasks, generate synthetic data to fix edge cases, and safely iterate through thousands of failures in isolated sandboxes. The core engine includes the sandbox runtime, policy engine, gateway (with k3s harness), privacy router, and CLI.
+[OpenShell](https://github.com/NVIDIA/OpenShell) is the runtime environment for autonomous agents -- the infrastructure where they live, work, and verify. It provides a programmable factory where agents can generate synthetic data to fix edge cases and safely iterate through thousands of failures in isolated sandboxes. The core engine includes the sandbox runtime, policy engine, gateway (with k3s harness), privacy router, and CLI.
 
 This repo is the community ecosystem around OpenShell -- a hub for contributed skills, sandbox images, launchables, and integrations that extend its capabilities. For the core engine, docs, and published artifacts (PyPI, containers, binaries), see the [OpenShell](https://github.com/NVIDIA/OpenShell) repo.
 
@@ -18,7 +18,6 @@ This repo is the community ecosystem around OpenShell -- a hub for contributed s
 | `sandboxes/base/`       | Foundational image with system tools, users, and dev environment |
 | `sandboxes/sdg/`        | Synthetic data generation workflows                          |
 | `sandboxes/openclaw/`   | OpenClaw -- open agent manipulation and control              |
-| `sandboxes/simulation/` | General-purpose simulation sandboxes                         |
 
 ## Getting Started
 
@@ -45,7 +44,7 @@ After the Brev instance is ready, access the Welcome UI to inject provider keys
 openshell sandbox create --from openclaw
 ```
 
-The `--from` flag accepts any sandbox defined under `sandboxes/` (e.g., `openclaw`, `sdg`, `simulation`), a local path, or a container image reference.
+The `--from` flag accepts any sandbox defined under `sandboxes/` (e.g., `openclaw`, `sdg`), a local path, or a container image reference.
 
 ## Contributing