feat(gateway): add ignored_threads config for Telegram

[teknium1]

Summary

Adds ignored_threads config for Telegram so bots can silently ignore specific forum topics in supergroups before any message processing happens. Messages from ignored thread IDs are dropped in _should_process_message() before any agent work — zero LLM cost, zero noise.

Salvaged from #9348 by @Jiawen-lee (cherry-picked onto current main with original authorship preserved).

Fixes #9208

Changes

• gateway/platforms/telegram.py — _telegram_ignored_threads() helper + early-return check in _should_process_message()
• gateway/config.py — bridges top-level telegram.ignored_threads to TELEGRAM_IGNORED_THREADS env var
• tests/gateway/test_telegram_group_gating.py — tests for feature + config bridging

Config

platforms:
  telegram:
    extra:
      ignored_threads:
        - 31
        - 42

Or top-level:

telegram:
  ignored_threads:
    - 31
    - 42

Test results

tests/gateway/test_telegram_group_gating.py: 8 passed
tests/gateway/: 2827 passed, 16 failed (pre-existing, unrelated)

[haanho]

Great addition! However, I think adding support for ignored_threads inside the group_topics list would be even more powerful for those managing multiple supergroups. It would avoid blocking the same thread ID across different chats. What do you think about moving the check to be chat-id aware?