hermes - 💡(How to fix) Fix Add free_response_channels config for Slack

Add a free_response_channels config option (similar to Discord's existing feature) to allow the bot to respond in designated Slack channels without requiring an explicit @bot mention.

Use case: Users often have project-specific Slack channels where they want the bot to participate conversationally without requiring a mention on every message. This is especially useful in small private channels where the bot is the only non-human participant.

Proposed config:

gateway:
  slack:
    free_response_channels: "C0123456789, C0987654321"  # comma-separated channel IDs
    # OR
    free_response_channels: "ALL"  # respond in all channels without mention

Behavior:

• In a free-response channel, any non-bot message triggers the LLM.
• The bot still respects reply_in_thread semantics.
• Thread replies (real thread_ts, not synthetic) continue to work normally.
• This is purely additive; existing @mention logic is unchanged.

(Maintainers: please add enhancement / platform: slack labels if available.)