feat(discord): add per-channel auto-thread exclusion list

[Helmi]

Summary

Adds a discord.no_auto_thread_channels config option that lets users exclude specific channels from auto-threading while keeping it enabled globally.

Motivation

When discord.auto_thread is enabled, every @mention in a text channel spawns a new thread. This is great for most channels (keeps conversations isolated), but some channels — like shared collaboration channels with multiple bots or group discussion channels — work better without auto-threading.

Currently the only option is to disable auto_thread globally, which is all-or-nothing.

Changes

• gateway/platforms/discord.py: Check DISCORD_NO_AUTO_THREAD_CHANNELS env var before creating auto-threads. If the message's channel ID is in the exclusion list, skip thread creation.
• gateway/config.py: Bridge discord.no_auto_thread_channels from config.yaml to the DISCORD_NO_AUTO_THREAD_CHANNELS env var. Supports both YAML list and comma-separated string formats (same pattern as free_response_channels).
• tests/gateway/test_discord_free_response.py: Two new tests — one verifying excluded channels skip auto-threading, one verifying non-excluded channels still auto-thread normally.

Usage

# config.yaml
discord:
  auto_thread: true
  no_auto_thread_channels:
    - '1489269514184032318'  # shared bot collaboration channel

Or via env var:

DISCORD_NO_AUTO_THREAD_CHANNELS=1489269514184032318,1234567890

Tests

All 17 tests in test_discord_free_response.py pass (including 2 new ones).

[teknium1]

Thanks for the contribution! This functionality is already available on main via discord.no_thread_channels (and the DISCORD_NO_THREAD_CHANNELS env var), which was added after this PR was opened.

Evidence:

• gateway/config.py lines 652–657 — bridges discord.no_thread_channels from config.yaml to DISCORD_NO_THREAD_CHANNELS, supporting both YAML list and comma-separated string (same pattern as free_response_channels)
• gateway/platforms/discord.py lines 3251–3256 — reads DISCORD_NO_THREAD_CHANNELS and sets skip_thread = True for matching channel IDs before the auto_thread branch fires, producing identical behavior to what this PR implements
• Introduced by commit f6d4b6a3198b35e2ee340c617b2f3193a9ab727b (feat(discord): add ignored_channels and no_thread_channels config)

Usage with the existing feature:

discord:
  auto_thread: true
  no_thread_channels:
    - '1489269514184032318'

This is an automated hermes-sweeper review.