Handling of optional output fields in limactl-mcp

When looking at the output schema, I find that all optional output fields (except strings) are defined as pointers. Looking at the `.outputSchema` property for the `run_shell_command` tool:

```json
{
"type": "object",
"required": [
"stdout",
"stderr"
],
"properties": {
"error": {
"type": "string",
"description": "Any error message reported by the subprocess."
},
"exit_code": {
"type": [
"null",
"integer"
],
"description": "Exit code of the command."
},
"stderr": {
"type": "string",
"description": "Output from the standard error stream."
},
"stdout": {
"type": "string",
"description": "Output from the standard output stream."
}
},
"additionalProperties": false
}
```

`exit_code` has a type of `null`, or `integer`, but that is kind of redundant because it is not a required output fields.

But `error` is not a required output field either, and it is just `string`, and not `null`, or `string`, which feels inconsistent.

But ignoring this, why is `exit_code` not a required output value? Does the tool ever return before the shell command exits? How can you not have an output value?

---

I think the description of `error` is also confusing, as it is not an error reported by the subprocess, but an error reported by the MCP because running the subprocess failed.