Add draft RFC for vMCP and proxyrunner horizontal scaling

Introduces THV-XXXX covering background, problems, scope, high-level
solution, and requirements for enabling safe horizontal scale-out of
the vmcp and thv-proxyrunner components via externalized Redis session
storage and session-aware routing.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: jerm-dro