[Task]: Add forwarded message author context to Telegram prompts

[crankshift]

Summary

Implement Telegram forwarded message author support so OpenCode receives clear author context for forwarded messages. If Telegram does not provide a forwarded author, treat the authorized Telegram user as the message author.

Context

Telegram can include original author information when a user forwards a message to the bot. The gateway should preserve that attribution when sending prompts to OpenCode so the AI can distinguish between the person forwarding the message and the original author. The implementation should keep Telegram-specific details inside the Telegram adapter and pass messenger-neutral author context into core prompt handling.

Acceptance Criteria

• Forwarded Telegram messages include safe author context in the prompt sent to OpenCode.
• Non-forwarded messages default to the authorized Telegram user as the author.
• Missing, hidden, or unavailable forwarded author data falls back safely without leaking raw Telegram payloads.
• Core gateway interfaces remain messenger-neutral and do not expose grammY/Telegram types.
• Tests cover forwarded messages with author data, forwarded messages without author data, and normal user-authored messages.

Notes

Keep user-facing and persisted data safe. Do not store raw Telegram payloads or private identifiers unless there is a concrete need.