docs: clarify that malicious config files are not an attack vector

Author: thdxr

SECURITY.md

@@ -24,6 +24,7 @@ Server mode is opt-in only. When enabled, set `OPENCODE_SERVER_PASSWORD` to requ
 | **Sandbox escapes**             | The permission system is not a sandbox (see above)                      |
 | **LLM provider data handling**  | Data sent to your configured LLM provider is governed by their policies |
 | **MCP server behavior**         | External MCP servers you configure are outside our trust boundary       |
+| **Malicious config files**      | Users control their own config; modifying it is not an attack vector    |
 
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