fix(discord): skip auto-threading reply messages

[OwenYWT]

Summary

• skip Discord auto-thread creation for MessageType.reply messages
• keep quote-replies in their original free-response channel so they still dispatch to the agent
• read require_mention and free_response_channels from config.extra before falling back to env vars
• add regression coverage for reply messages and config-backed free-response channels

Root cause

Discord reply messages can be accepted by the message filter, but _handle_message still tried to auto-create a thread for them. Reply-based conversations should stay in the original channel, and thread creation can fail for reply messages.

Test plan

• python -m pytest -o addopts='' tests/gateway/test_discord_free_response.py tests/gateway/test_discord_channel_controls.py -q
• python -m py_compile gateway/platforms/discord.py tests/gateway/test_discord_free_response.py
• python -m pytest -o addopts='' tests/gateway/test_discord_system_messages.py tests/gateway/test_discord_slash_commands.py::test_auto_thread_skips_threads_and_dms -q
• git diff --check

Related: #9399

[RuckVibeCodes]

[gus-first-pass] fix(discord): skip auto-threading reply messages - Clear implementation, no issues found.

[teknium1]

Merged via #11629 — #11629

Your commit (95b43c17) was cherry-picked with authorship preserved — both the is_reply_message skip and the _discord_require_mention / _discord_free_response_channels helpers with config.extra fallback are in.

Thanks for the clean scope (no unrelated bundled changes), the config.extra robustness improvement (consistent with channel_skill_bindings), and the regression coverage (test_discord_reply_message_skips_auto_thread + test_discord_free_response_channel_can_come_from_config_extra). Closes #9399 along with @Hypn0sis's free-response-skip from PR #9650 — your reply-skip addressed the other half of that bug.