Add a command to save object_info for offline validation

**Describe the solution you'd like**

Could we add `comfy nodes snapshot --output object_info.json`?

It should use the normal target settings, stream the response, validate the JSON, and write the file atomically. This would give MCP clients a clean way to cache large node catalogs for offline validation.

**Describe alternatives you've considered**

Today users need to call `/object_info` with curl and manage the file manually.

**Additional context**

This gives comfy-mcp a CLI command it can wrap without adding direct HTTP logic.

Start by tracing the existing `comfy nodes` command entry points and how normal target settings are applied. Use the `/object_info` request as the source, then verify that the command streams and validates the JSON before atomically producing the requested `object_info.json` file.

Written by the indexing model from the issue text.