Telegram channel plugin: forum Topics support (message_thread_id in channel meta + reply tool)

## Feature request for the official telegram channel plugin

Telegram supergroups with **Topics** (forums) are a natural UI for multi-session workflows: one topic per Claude Code session, the topics list as a session browser. The official telegram plugin currently makes this impossible:

- inbound messages drop `message_thread_id`, so the session cannot tell which topic the user wrote in;
- the `reply` tool has no way to post into a topic — answers land in General.

## Proposed change

Tiny and backward compatible (+14/−4 in server.ts):
1. include `message_thread_id` in the `` meta when `is_topic_message`;
2. accept optional `message_thread_id` in `reply` and pass it to sendMessage/sendPhoto/sendDocument;
3. one sentence in the instructions text telling the model to echo the thread id back.

A ready patch exists and has been running in production for days against a live bot (macOS/bun; verified topic routing incl. chunked replies and attachments, DM behavior unchanged): https://github.com/ipauler/claude-plugins-official/commit/f777c0c49a5eae431aeca4cf08564a77f007903e — it was submitted as anthropics/claude-plugins-official#5391 and auto-closed by policy (team-members-only repo), so filing it here as requested by the bot.

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Start in server.ts and review the linked commit for the Telegram channel plugin's inbound metadata and reply handling. Check the sendMessage, sendPhoto, and sendDocument paths, then verify that topic replies and attachments preserve message_thread_id while direct-message behavior remains unchanged.

Vom Indexierungsmodell aus dem Issue-Text verfasst.