0.56.3: Button Matrices and Custom-Message Starts

• Button grammar: Adjacent complete JSON/CML cells and matrices now compose one button surface in source order instead of dropping fenced controls or truncating footer controls. Each matrix contributes its existing rows; horizontal grouping, disabled controls and callback semantics remain unchanged. Invalid fenced groups register no callbacks. This extends button syntax only; rendering mode and voice syntax are unchanged.
• Pi compatibility: Extension-triggered runs on Pi 1.2 no longer fail when before_agent_start carries a custom message without a user prompt. Custom starts retain connected bridge guidance without being mistaken for fresh Telegram user input; legacy user events, unavailable-transport filtering and ordered system-prompt blocks remain supported.