FAQ: migration and adoption guide (post-publish)

Track frequently asked questions and gotchas as teams adopt RDCP:\n\n- Installation and peer deps (frameworks)\n- Server adapter setup (Express/Fastify/Koa)\n- Auth configuration across Basic/Standard/Enterprise\n- Client SDK usage patterns and error handling\n- Common validation errors and fixes\n\nWe will consolidate into a wiki FAQ section once patterns stabilize.