Replace deprecated SSE transport with Streamable HTTP (MCP spec 2025-03-26). Add OptionalAuthMiddleware for agent Bearer token auth on /mcp endpoint — authenticates when token present, passes through for unauthenticated tools like register_agent. Also fix .gitignore to only ignore root synapbus binary, not cmd/synapbus source directory. Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>