feat(telegram): add outbound notification policy

[tuchg]

Summary

• Add a shared channel NotificationPolicy for text, fallback files, and agent attachments.
• Map silent text/file/photo/video/document Telegram sends to disable_notification(true).
• Mark live progress/drafts/history replay silent, then send final assistant output as a new notify-eligible message and delete the silent preview after the final send succeeds.
• Keep user-action prompts and final attachments notify-by-default.
• Record the UX/boundary decision under docs/decisions/.

Closes #35.

Verification

• cargo +nightly test -Zbuild-dir-new-layout -p lucarne-channel
• cargo +nightly test -Zbuild-dir-new-layout -p lucarne-telegram --lib turn::tests::
• cargo +nightly test -Zbuild-dir-new-layout -p lucarne-telegram --lib
• cargo +nightly check -Zbuild-dir-new-layout --workspace --all-features