feat(discord): semantic thread titles for auto-created threads

[rungmc357]

Summary

When Hermes auto-creates a Discord thread from a mention, the thread gets a
fast placeholder name derived from the raw message content (first 80 chars
with mention syntax stripped). This is the same pattern Telegram already has
for forum topics — but Discord threads keep that placeholder forever.

This PR adds the Discord equivalent of the existing Telegram
_rename_telegram_topic_for_session_title flow:

1. _auto_create_thread tags the created thread with
   _hermes_auto_thread_initial_name (the placeholder name)
2. SessionSource gains auto_thread_created and
   auto_thread_initial_name fields so the gateway can distinguish
   Hermes-created threads from pre-existing or human-renamed ones
3. After the first agent turn, when the auto-title generator produces a
   session title, the gateway schedules a best-effort
   adapter.rename_thread() that renames only threads whose current name
   still matches the original placeholder — so human-renamed threads are
   never overwritten
4. rename_thread on the Discord adapter is guarded with
   only_if_current_name to prevent clobbering

This mirrors the Telegram topic rename path (_is_telegram_topic_lane →
_schedule_telegram_topic_title_rename →
_rename_telegram_topic_for_session_title) and reuses the same
title_callback hook in maybe_auto_title.

Changes

• gateway/session.py: Add auto_thread_created /
  auto_thread_initial_name to SessionSource (dataclass + serialization)
• gateway/platforms/base.py: Pass new fields through _make_session_source
• plugins/platforms/discord/adapter.py: Extract _derive_auto_thread_name,
  tag created threads with initial name, add rename_thread method with
  only_if_current_name guard
• gateway/run.py: Add _is_discord_auto_thread_lane,
  _sanitize_discord_thread_title,
  _rename_discord_auto_thread_for_session_title,
  _schedule_discord_semantic_thread_rename; wire the title callback for
  Discord auto-thread lanes
• tests/gateway/test_discord_slash_commands.py: Tests for auto-thread
  config bridge
• tests/gateway/test_fast_command.py: Tests for fast-command dispatch on
  auto-thread lanes

Test plan

python -m pytest tests/gateway/test_discord_slash_commands.py \
  tests/gateway/test_fast_command.py -o 'addopts=' -q

All 43 tests pass.

[teknium1]

Merged via #60187 - your commit was cherry-picked onto current main with your authorship preserved in git log, plus a small follow-up making the title sanitizers UTF-16-safe. Your implementation was chosen from six competing PRs for having the right scoping guard (auto_thread_created + only_if_current_name) at the right size. Thanks!