Telegram has a 4096 character limit per message. This fix:
- Splits messages longer than 4000 chars into multiple chunks
- Prefers breaking at newline boundaries to preserve formatting
- Falls back to space boundaries if no newlines available
- Forces split at max length if no good boundaries exist
- Adds comprehensive tests for message splitting logic
- Replaces fragile input() hacks with robust prompt_toolkit.PromptSession
- Native support for multiline paste, history, and clean display
- Restores animated spinner in _thinking_ctx (now safe)
- Replaces boxed Panel with clean header for easier copying
- Adds prompt-toolkit dependency
- Adds new unit tests for input layer