Adds the experimental `docker-chat-sandbox` example template under
`examples/templates/x/`. It provisions a regular dev agent plus a
chat-designated agent that runs inside bubblewrap with a read-only root,
writable `/home/coder`, and outbound TCP restricted to the Coder
control-plane endpoint via `iptables`.
The chat agent still appears in dashboard and API responses, but the
template reserves it for chatd-managed sessions rather than normal user
interaction. `lint/examples` now walks nested template directories, so
experimental templates can live under `examples/templates/x/` without
treating `x/` itself as a template.