Pi core has no MCP client. V2 already minted a scoped t3-code bearer
before openSession. This writes a T3-owned extension into the server
cache and spawns pi --mode rpc --extension <that file> with T3_MCP_URL
and T3_MCP_BEARER_TOKEN. Each MCP tool is registered under its original
name. User launchArgs are preserved. The first turn receives the shared
orchestration instructions.