Add Telegram message timestamp metadata to forwarded prompts

What:
- Prefix prompt text sent to OpenCode with Telegram send-time metadata when  is available
- Apply the metadata wrapping consistently through text, photo, and document prompt flows
- Extend handler tests to assert metadata forwarding and add a changeset for release

Why:
- The model needs reliable temporal context to reason correctly about delayed or out-of-order delivery scenarios
- Including the original Telegram timestamp improves response quality without changing session routing or event handling