docs: clarify that the Docker terminal backend is a single persistent container (#20003)

The docs were ambiguous about whether the Docker terminal backend spins up
a fresh container per command or reuses a long-lived one. It's the latter
— Hermes starts one container on first use and routes every terminal,
file, and execute_code call through docker exec into that same container
for the life of the process (across /new, /reset, and delegate_task
subagents). Working-directory changes, installed packages, and files in
/workspace persist from one tool call to the next, like a local shell.

- configuration.md: lead the Docker Backend section with the persistence
  model before the YAML example; sharpen the Backend Overview table row.
- features/tools.md: expand the Docker Backend block (previously just a
  2-line YAML stub) with a clear statement of the persistent-container
  semantics and a pointer to the full lifecycle section.
- docker.md: tighten the 'Docker as a terminal backend' bullet and the
  'Skills and credential files' paragraph to call out the single-container
  model explicitly.

website/docs/user-guide/features/tools.md

@@ -84,6 +84,10 @@ terminal:
   docker_image: python:3.11-slim
 ```
 
+**One persistent container, shared across the whole process.** Hermes starts a single long-lived container on first use (`docker run -d ... sleep 2h`) and routes every terminal, file, and `execute_code` call through `docker exec` into that same container. Working-directory changes, installed packages, environment tweaks, and files written to `/workspace` all carry over from one tool call to the next, across `/new`, `/reset`, and `delegate_task` subagents, for the lifetime of the Hermes process. The container is stopped and removed on shutdown.
+
+This means the Docker backend behaves like a persistent sandbox VM, not a fresh container per command. If you `pip install foo` once, it's there for the rest of the session. If you `cd /workspace/project`, subsequent `ls` calls see that directory. See [Configuration → Docker Backend](../configuration.md#docker-backend) for the full lifecycle details and the `container_persistent` flag that controls whether `/workspace` and `/root` survive across Hermes restarts.
+
 ### SSH Backend
 
 Recommended for security — agent can't modify its own code: