Track C's enable-mother wrote external-mcp.json by overwriting the whole file
with a single mother server. Use jq to merge the mother entry into the existing
registry so any other configured servers are preserved, and write atomically
(mktemp in the same dir + mv). This is the concrete consumer that makes jq a
real dependency of the MCP path. Fails loudly if jq is absent.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
