Add forwarded message author context to Telegram prompts

[crankshift]

Summary

• Adds safe Telegram author extraction for forwarded user, hidden-user, chat, and channel origins.
• Passes messenger-neutral author context through text, photo, album, and voice prompts before sending to OpenCode.
• Documents forwarded author attribution behavior and fallback safety.

Test Plan

• pnpm run lint
• pnpm test
• pnpm run check

Closes #19