docs: comprehensive Discord guide (rename + extend discord-bot-howto.md)

[thepagent]

Description

Renames docs/discord-bot-howto.md to docs/discord.md and extends it from a basic setup guide to a comprehensive Discord reference.

New sections:

• Configuration reference — all Discord config options with tables
• @Mention behavior — user mention vs role mention (role mentions ignored since feat: per-thread serialization, conditional streaming, and multi-bot fixes #420)
• Thread behavior — involved vs mentions mode explained
• Multi-bot setup — same channel, known limitations, Helm example
• Bot-to-bot communication — allow_bot_messages + trusted_bot_ids
• Troubleshooting — Gateway drops (bug(discord): rollout restart causes silent Gateway session conflict — bot stops receiving events #455), thread creation failures (bug(discord): @mentioned bot ignored in another bot's thread — multi-bot same-channel #457), auth issues

Also updates the README link from discord-bot-howto.md to discord.md.

References: #420, #440, #441, #455, #457

[github-actions]

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