feat(telegram): add topic group trigger overrides

[bogdanovich]

Summary

Adds per-topic group_trigger overrides for Telegram forum topics.

This lets a bot stay conservative in a Telegram supergroup by default, for example mention_only: true, while being active by default in one dedicated forum topic:

{
  "channel_list": {
    "telegram": {
      "group_trigger": {
        "mention_only": true,
        "topics": {
          "1771": { "mention_only": false }
        }
      }
    }
  }
}

Why this is needed

Telegram supergroups often use forum topics as separate sub-channels. A bot owner may want the bot to be mention-only in most topics to avoid interrupting normal group conversation, but fully active in a dedicated topic where users expect the bot to answer every message.

Doing this in the prompt is unreliable because the message has already reached the agent. The right layer is the channel trigger filter, before the inbound message is published to the agent bus.

Design

• Extends existing group_trigger with optional topics map.
• Keeps the trigger logic in BaseChannel, so Telegram does not duplicate mention/prefix filtering.
• Telegram passes message_thread_id as the topic ID only for forum chats (is_forum=true), matching the existing session isolation behavior.
• Topic entries replace the channel-level group_trigger for that topic.

The replacement semantics are intentional. Current config uses plain bools, so a partial merge cannot distinguish an omitted mention_only from an explicit mention_only: false. A topic entry such as { "mention_only": false } should be an explicit permissive override.

Testing

• go test ./pkg/channels ./pkg/channels/telegram ./pkg/config