Track C's enable-mother overwrote external-mcp.json with a single mother
server. Use jq to merge the mother entry into the existing registry so other
configured servers are preserved, written atomically (mktemp in same dir + mv).
This is the concrete consumer that makes jq a real dependency of the MCP path;
fails loudly if jq is absent.
(Re-applied: the original commit was lost to a branch-recreation race when #97
merged at the packages-only commit.)
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
