Telegram and Resend outbound notification channels

[Ryan-Atkinson87]

The two outbound notification channels behind the #41 abstraction: Telegram (default, instant) and Resend email (optional, summaries).

Spec: §9.2 Notification channels, §10 Credentials, §16.3 (secrets).

Acceptance criteria

• Telegram outbound channel sends alerts to the human's chat via the bot (token + chat id resolved by reference) (§9.2, §10).
• Resend email channel sends digests/summaries; off by default and suited to summaries rather than instant alerts (§9.2).
• Both implement the Notification service: channel abstraction, dispatcher, config toggles #41 channel interface and register with the dispatcher.
• Each send has explicit success and failure paths; transient failures are surfaced, not swallowed.
• Secrets telegram_bot_token, telegram_chat_id, resend_api_key resolved by reference and added to .env.example (names only).
• Tests with the Telegram and Resend HTTP layers mocked.

Depends on: #41, #5