Adds stdio external MCP server registry support to colibri-mcp with read-only discovery by default and explicit COLIBRI_MCP_EXTERNAL_CALL gating for proxying external tools. Also smooths the merged jail-spawn formatting/FreeBSD command parameter edge so repository gates pass.\n\nChecks: cargo test -p colibri-mcp --all-targets; cargo fmt --check; ./scripts/check-format.sh; git diff --check; fake stdio MCP server smoke via colibri-mcp --external-config --external-call
