hermes - 💡(How to fix) Fix feat(telegram): auto-create forum topic on new session (parity with MATRIX_AUTO_THREAD)

Feature Description

Auto-create a forum topic in Telegram supergroups when a new conversation session starts, and route all messages for that session into the topic. This keeps the group clean — each conversation gets its own isolated thread, just like Slack/Discord threads.

Motivation

Currently, when a user starts a new /new chat in a Telegram group with topics enabled, the conversation stays in whatever topic the /new command was sent from. This means:

• Multiple concurrent conversations all pile into the same topic
• Context switching between conversations is hard — messages interleave
• The group becomes noisy and hard to follow

Matrix already has MATRIX_AUTO_THREAD (env var, default true) that does exactly this — on new session, it creates a thread and routes messages there. Telegram should have parity.

Proposed Solution

Add a TELEGRAM_AUTO_TOPIC env var (or extra.auto_topic config key), defaulting to false for backward compatibility.

When enabled and the chat is a supergroup with topics enabled:

1. On new session — call Telegram Bot API createForumTopic with a name like "Chat 2026-05-19 02:30" (timestamp-based).
2. Store the returned message_thread_id in session metadata.
3. Route all subsequent messages for that session using message_thread_id in sendMessage.
4. On session end/reset — optionally call closeForumTopic to archive the topic.

Reference Implementation

Matrix platform already does this at gateway/platforms/matrix.py:1714:

# Auto-thread.
if not thread_id and ((not is_dm and self._auto_thread) or (is_dm and self._dm_auto_thread)):
    thread_id = event_id
    self._threads.mark(thread_id)

For Telegram, the equivalent would be in the message handler, calling createForumTopic before session creation.

Alternatives Considered

• Manual topic creation — user creates topics manually before starting chats. Too much friction.
• Reuse existing topics — always send to a single "Hermes" topic. Doesn't solve the interleaving problem.
• No change — accept the noise. Suboptimal UX for topic-enabled groups.

Additional Context

• Telegram Bot API supports createForumTopic, editForumTopic, closeForumTopic, reopenForumTopic, deleteForumTopic since Bot API 6.3
• The gateway already has full thread/topic support for delivery (telegram:chat_id:thread_id format in send_message)
• DM Topics (extra.dm_topics) already exist for private chats — this extends the concept to group topics
• The _GENERAL_TOPIC_THREAD_ID = "1" constant already exists in telegram.py for the General topic