_save_platform_tools() overwrote the entire platform_toolsets list with
only the toolsets known to CONFIGURABLE_TOOLSETS. This silently dropped
any MCP server toolsets that users had added manually to config.yaml.
Fix: collect any existing toolset keys that are not in CONFIGURABLE_TOOLSETS
and append them back after the wizard's selections are written. This ensures
MCP toolsets survive a hermes tools save.
Fixes#1247
