Merge PR #451: feat: Add Daytona environment backend

Authored by rovle. Adds Daytona as the sixth terminal execution backend
with cloud sandboxes, persistent workspaces, and full CLI/gateway integration.
Includes 24 unit tests and 8 integration tests.

CONTRIBUTING.md

@@ -155,7 +155,7 @@ hermes-agent/
 │   ├── skill_tools.py            # Skill search, load, manage
 │   └── environments/             # Terminal execution backends
 │       ├── base.py                   # BaseEnvironment ABC
-│       ├── local.py, docker.py, ssh.py, singularity.py, modal.py
+│       ├── local.py, docker.py, ssh.py, singularity.py, modal.py, daytona.py
 │
 ├── gateway/                  # Messaging gateway
 │   ├── run.py                    # GatewayRunner — platform lifecycle, message routing, cron