feat(discord): add strict mention mode

[FatPigeorz]

Summary

• add discord.strict_mention / DISCORD_STRICT_MENTION to mirror Slack's accepted strict-mention pattern
• default to the current Discord behavior (strict_mention: false): previously participated threads can continue without repeated mentions
• when strict_mention: true, require an explicit Discord bot mention for every server channel/thread message unless the channel is free-response
• bridge the YAML config key into adapter config and env state without overriding an existing env var
• document the new Discord option and add regression coverage for multi-bot strict routing

Background

Discord already tracked threads where the bot participated and used that state to skip mention checks for follow-up messages. That behavior was unconditional. This PR keeps it as the default for backward compatibility, while adding the same style of opt-in strict mode that Slack added in #16193 after #8019.

This matters for multi-bot / multi-agent Discord servers: explicit mentions should be able to route a turn to one bot without waking another bot just because it participated earlier in the same thread.

Closes #16725

Behavior matrix

Context	Default (strict_mention: false)	strict_mention: true
DM	Responds	Responds
Free-response channel	Responds	Responds
Channel/thread, explicit @mention	Responds	Responds
Previously participated thread, no mention	Responds	Ignores until re-mentioned
Unknown thread/channel, no mention	Ignores	Ignores

Test Plan

• /root/.hermes/hermes-agent/venv/bin/python -m pytest tests/gateway/test_discord_free_response.py tests/gateway/test_config.py tests/gateway/test_discord_thread_persistence.py tests/gateway/test_discord_channel_controls.py -q
• /root/.hermes/hermes-agent/venv/bin/python -m pytest tests/gateway/test_discord_free_response.py tests/gateway/test_config.py tests/gateway/test_discord_thread_persistence.py tests/gateway/test_discord_channel_controls.py tests/gateway/test_slack_mention.py -q

[FatPigeorz]

@teknium1 could you take a quick look when you have a chance?

This mirrors the already-merged Slack strict_mention behavior from #16193 / #8019, keeps Discord backward-compatible by default, and only changes routing when discord.strict_mention: true is set for multi-bot / multi-agent servers.

Summary of intended behavior:

• default remains backward-compatible: discord.strict_mention: false
• strict mode is opt-in for multi-bot / multi-agent Discord servers
• when enabled, participated threads no longer bypass explicit mention routing
• free-response channels and direct mentions still work as before
• regression tests cover both the default path and strict path

Happy to adjust naming, scope, or tests if you prefer a different shape for the Discord adapter.

[teknium1]

Thanks for the focused multi-bot Discord routing work. This is now implemented on current main under the shipped name discord.thread_require_mention / DISCORD_THREAD_REQUIRE_MENTION.

• plugins/platforms/discord/adapter.py:6206-6214 disables the known-thread mention bypass when thread_require_mention is enabled, while retaining the existing default continuation behavior.
• plugins/platforms/discord/adapter.py:8255-8258 bridges YAML without overriding an explicit environment setting; tests/gateway/test_config.py:564-599 covers both cases.
• tests/gateway/test_discord_free_response.py:724-774 covers the known-thread strict gate, direct mentions, and YAML/config-extra behavior.
• This behavior shipped in d863773c81b4d1c958b2f28b76bcd8b0809d7eac (feat(discord): add thread_require_mention for multi-bot threads) and is present in v2026.5.16.
• This also matches the maintainer's verification on linked issue Discord known-thread participation bypass cannot be disabled #16725.

Automated hermes-sweeper review.