docs: clarify workspace vs sandbox

docs/concepts/agent-workspace.md

@@ -12,6 +12,11 @@ file tools and for workspace context. Keep it private and treat it as memory.
 This is separate from `~/.clawdbot/`, which stores config, credentials, and
 sessions.
 
+**Important:** the workspace is the **default cwd**, not a hard sandbox. Tools
+resolve relative paths against the workspace, but absolute paths can still reach
+elsewhere on the host unless sandboxing is enabled. If you need isolation, use
+[`agent.sandbox`](/gateway/sandboxing) (and/or per‑agent sandbox config).
+
 ## Default location
 
 - Default: `~/clawd`