fix: add Discord threaded free-response channels

[itsflownium]

Summary\n- Add discord.free_response_thread_channels / DISCORD_FREE_RESPONSE_THREAD_CHANNELS for mention-free Discord channels that still use automatic threading.\n- Keep existing free_response_channels behavior inline by skipping thread creation only for the original list.\n- Apply the new setting to the early human-mention gate, while retaining mention filtering in all other channels.\n- Add Discord adapter regression tests and document the setting.\n\nValidation\n- 80 focused Discord tests\n- Ruff on the changed adapter and tests\n\nFixes #52766

[tonydwb]

Code Review Summary

Verdict: Approved

Looks Good

• Clean extraction of _discord_channel_id_set() as a reusable static method
• New _discord_free_response_thread_channels() follows the same pattern
• Proper config + env var fallback chain
• Well-documented with clear docstrings
• Feature is additive — no breaking changes to existing behavior

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Reviewed by Hermes Agent

[teknium1]

Thanks for targeting a real Discord routing gap: current main still makes is_free_channel imply skip_thread at plugins/platforms/discord/adapter.py:6223.

Problems

• The new setting is only consumed in _handle_message. The earlier on_message gate admits human-mentioned, bot-unmentioned messages only when _discord_free_response_channels() matches (plugins/platforms/discord/adapter.py:1201-1204), so the proposed threaded-free setting would not consistently be mention-free.
• The adapter hunk is stale: gh pr diff 52802 | git apply --check --verbose fails at the former helper block. Current routing uses _discord_channel_keys() (plugins/platforms/discord/adapter.py:6170), which the port should preserve.

Suggested changes

• Rebase the logic conceptually onto the current channel-key flow: use the new set for mention gating, but retain threading unless the channel is in the existing inline-free or no-thread sets.
• Extend the early delivery gate and add a regression case for a threaded-free channel containing a human mention without a Hermes mention.

Automated hermes-sweeper review.