Slack, Discord and WeChat default traffic through the same binding registration #1112

Parent: #1107 · ADR-0142 · moved up right after #1108 (Vain, 2026-10-07: after Lark/Feishu, do Slack, WeChat and Discord next)

What to build

• Host:
  • Acquire the account-level registration for every Binding, not only Lark. Lark and Feishu share the feishu channel, so Tracer: bind a connected Lark app; DMs and mentions reach the Inbox; the reply goes back in place #1108 already covers Lark.
  • Admit default traffic through it:
    • Slack: DMs and channel @mentions.
    • Discord: DMs, plus guild-channel and thread @mentions.
    • WeChat: only DMs from the paired owner that the Provider lists. Other contacts and groups are never admitted.
  • DMs are admitted when the Provider delivers them; the platform decides reach.
  • Implicit entries, auto/ask and dedupe work as in Tracer: bind a connected Lark app; DMs and mentions reach the Inbox; the reply goes back in place #1108.
• Client:
  • Bind app shows the real receiving state for every platform.
  • WeChat's ready message says that only the scanner's DMs arrive.

Acceptance criteria

• Slack: bind → DM or @mention in a channel the app is in → Inbox → reply in the same DM or thread.
• Discord: bind → DM or @mention in a guild channel or thread → Inbox → reply in place.
• WeChat: bind → the owner's DM → Inbox → reply. A stranger's DM and group messages are not admitted.
• A recorded group follows the platform's global group-intake and wake defaults.
• Automated coverage plus zh/en, light/dark screenshots. Real-platform verification by Vain.

Follow-ups

• Block, mute and per-conversation rules come in Conversation list: mute, rules, block that sticks, allow again #1109.
• Inline creation (Slack/Discord token forms, Discord consumerMode: external-consumer) comes in Create the app inside Bind app: Lark credentials and WeChat QR (Provider setup capability) #1111.