feat: support discord bot-to-bot interaction

[sinnce]

Summary\n- allow Discord bot messages through when they mention Hermes or reply to a Hermes message\n- add optional outbound bot mention support via config/env/metadata\n- cover the new behavior with Discord gateway tests\n\n## Testing\n- python -m pytest tests/gateway/test_discord_bot_filter.py tests/gateway/test_discord_send.py tests/gateway/test_discord_reply_mode.py -q

[teknium1]

Thanks for the Discord gateway work. There are blocking design and correctness issues on current main.

Problems

• The new bot-admission helper accepts an explicitly mentioned bot message before enforcing allow_bots == "none". That regresses the documented safe default, while current main rejects bot authors under none at plugins/platforms/discord/adapter.py:1130-1136.
• The proposed auto-mention/reply flow is the topology current Discord docs explicitly mark unsupported: reply-pings can satisfy another Hermes bot's mention gate indefinitely (website/docs/user-guide/messaging/discord.md:304-308).
• The patch targets gateway/platforms/discord.py, but current main moved the adapter to plugins/platforms/discord/adapter.py in cc8e5ec2afbfd10a3cff4e710210dd9ecae64a33; this requires a fresh port rather than a clean salvage.

Suggested changes

• Keep the none bot rejection intact and do not add automatic bot-to-bot reply/mention behavior without an explicit design decision.
• If a narrowly scoped trusted-relay case is desired, build on the current DISCORD_ALLOW_BOTS=mentions path in the plugin adapter and add plugin-targeted regression coverage.

Automated hermes-sweeper review.