feat(gateway): scope restart notifications to channel allowlists

[walshg3]

Summary

• Add optional per-platform gateway_restart_notification_channels allowlists for gateway lifecycle/restart notifications.
• Apply the allowlist anywhere restart lifecycle messages are emitted: active-session shutdown notices, home-channel shutdown notices, planned restart confirmations, and startup notifications.
• Bridge the key from both top-level platform config and nested gateway.platforms.<platform> config, and document the config example.

Default behavior is unchanged: if gateway_restart_notification_channels is unset, the platform behaves exactly as it did before. gateway_restart_notification: false remains the coarse per-platform kill switch.

Why

Multi-platform gateways often have several active chats, but restart lifecycle pings are operator noise. Users should be able to keep the signal in one ops/home channel without spraying Telegram, Discord, Slack, etc. chats that are not meant to receive operational lifecycle messages.

This is a narrower alternative/complement to the existing open restart-notification PRs:

• feat(gateway): add gateway_restart_notification_channels config #52584 uses gateway_restart_notification_channels as a boolean to suppress bare channel/group notices.
• feat(gateway): per-platform gateway_restart_notification_channel override #31046 routes restart notifications to one override channel.

This PR keeps the existing delivery targets, but gates them through an explicit chat/channel ID allowlist. That makes the behavior easy to reason about and keeps current behavior as the unset default.

Test plan

python -m pytest tests/gateway/test_restart_notification.py tests/gateway/test_config.py -q -o 'addopts='

Result: 109 passed, 2 warnings.

[walshg3]

Closing this draft because the related open PRs #52584 and #31046 look sufficient for this need. I’ll track those instead.