docs: add 'copilot' to the list of coding agents

Signed-off-by: Joshua Boelter <joshua.boelter@gmail.com>

Author: jboelter

sandboxes/base/README.md

@@ -9,7 +9,7 @@ The foundational sandbox image that all other OpenShell Community sandbox images
 | OS | Ubuntu 24.04 |
 | Languages | `python3` (3.13), `node` (22.22.1) |
 | Package managers | `npm` (11.11.0), `uv` (0.10.8), `pip` |
-| Coding agents | `claude`, `opencode`, `codex` |
+| Coding agents | `claude`, `opencode`, `codex`, `copilot` |
 | Developer | `gh`, `git`, `vim`, `nano` |
 | Networking | `ping`, `dig`, `nslookup`, `nc`, `traceroute`, `netstat`, `curl` |