Explain renders a Markdown description of the sandbox environment from cfg,
for the AI agent to read on demand via `agent-sandbox ai explain`. The prose
lives in explain.tmpl; this function only prepares the view data.
Pointer returns the short guidance injected into the agent's system prompt at
launch. It intentionally carries no allow/drop detail — only a pointer to the
live `agent-sandbox ai explain` command — to keep the system prompt small.