fix(agent): soften SSRF guard recovery

Keep private URL access blocked at the tool boundary, but return a clear non-retryable hint so the agent can recover conversationally instead of aborting the turn.

Co-authored-by: Cursor <cursoragent@cursor.com>

nanobot/agent/tools/shell.py

@@ -321,7 +321,7 @@ class ExecTool(Tool):
 
         from nanobot.security.network import contains_internal_url
         if contains_internal_url(cmd):
-            # SSRF stays fatal in the runner, so keep this marker direct.
+            # The runner turns this marker into a non-retryable security hint.
             return "Error: Command blocked by safety guard (internal/private URL detected)"
 
         if self.restrict_to_workspace: