New internal/masterdb/ package for mcp-master cluster state. Separate
from the agent's registry because the schemas are fundamentally
different (cluster-wide placement vs node-local containers).
Tables: nodes, placements, edge_routes. Full CRUD with tests.
Follows the same Open/migrate pattern as internal/registry/.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
