Add Discord as a messaging channel, alongside the existing Telegram and WhatsApp channels.

Split out of #76 (now superseded by #78) — Discord shares no code with the evomem/setup work, so it stands alone.

Change

DiscordChannel(BaseChannel) in backend/channels/discord.py — an async client on a daemon thread, mirroring the Telegram channel's structure. Listens to DMs and to guild messages that @mention the bot, routes them through the agent runtime, and replies in the originating channel. Reuses the shared pairing/allowlist flow, outbound buffering, typing indicator, and approval prompts (rendered as discord.ui buttons). Media: image vision, file send, attachment persistence — gated by the agent's attachment config and per-type size limits.

• Registered 'discord' in CHANNEL_TYPES (the channel registry is the only extension point).
• requirements.txt: discord.py>=2.3.
• doctor gains a channel-integrations check (verifies the library is present for configured channels; notes the required Message Content Intent).
• UI: a setup hint under the Discord bot-token field in templates/agent_detail.html.

Notes

• Requires the privileged Message Content Intent (enabled in the Discord Developer Portal) to read message text. The channel logs a clear error if it is missing.
• Discord renders Markdown, so responses are not stripped of formatting; messages are split to respect the 2000-char limit.
• Intentionally not added to the setup wizard — Discord is configured per-agent like the other channels.

Why it's safe

• Additive — new module + one registry entry; existing channels untouched. No models/ schema changes.
• The bot token is never logged; file sends are size- and access-checked before open.

Test plan

• test_discord_channel green (Discord client mocked).
• doctor --quick channel-integrations check runs cleanly.