feat(discord): auto-thread on @mention + skip mention in bot threads

Two changes to align Discord behavior with Slack:

1. Auto-thread on @mention (default: true)
   - When someone @mentions the bot in a server channel, a thread is
     automatically created from their message and the response goes there.
   - Each thread gets its own isolated session (like Slack).
   - Configurable via discord.auto_thread in config.yaml (default: true)
     or DISCORD_AUTO_THREAD env var (env takes precedence).
   - DMs and existing threads are unaffected.

2. Skip @mention in bot-participated threads
   - Once the bot has responded in a thread (auto-created or manually
     entered), subsequent messages in that thread no longer require
     @mention. Users can just type normally.
   - Tracked via in-memory set (_bot_participated_threads). After a
     gateway restart, users need to @mention once to re-establish.
   - Threads the bot hasn't participated in still require @mention.

Config change:
   discord:
     auto_thread: true  # new, added to DEFAULT_CONFIG

Tests: 7 new tests covering auto-thread default, disable, bot thread
participation tracking, and mention skip logic. All 903 gateway tests pass.

hermes_cli/config.py

@@ -280,6 +280,7 @@ DEFAULT_CONFIG = {
     "discord": {
         "require_mention": True,       # Require @mention to respond in server channels
         "free_response_channels": "",  # Comma-separated channel IDs where bot responds without mention
+        "auto_thread": True,           # Auto-create threads on @mention in channels (like Slack)
     },
 
     # Permanently allowed dangerous command patterns (added via "always" approval)