Found in a clean cps sandbox run: a machine without git passes the Node check
but fails later inside 'pi install git:...' with an unclear error. Now checked
up front with an actionable message. CHANGELOG updated.
Decision on optionalDependencies (@modelcontextprotocol/sdk): KEEP. The heavy
install observed in the sandbox (~317 packages) is dominated by npm>=7
auto-installing the Pi peerDependencies stack, not the MCP SDK; removing the
SDK saves little and would make MCP enablement fragile across pi updates.