Salvages PR #3005 by web3blind. Cherry-picked onto current main with functional skill binding and docs added.
- DM topic creation via createForumTopic (Bot API 9.4, Feb 2026)
- Config-driven topics with thread_id persistence across restarts
- Session isolation via existing build_session_key thread_id support
- auto_skill field on MessageEvent for topic-skill bindings
- Gateway auto-loads bound skill on new sessions (same as /skill commands)
- Docs: full Private Chat Topics section in Telegram messaging guide
- 20 tests (17 original + 3 for auto_skill)
Closes#2598
Co-authored-by: web3blind <web3blind@users.noreply.github.com>
36af1f3baf