fix: extract <at> tags as standalone post elements for real mentions

[iamkentzhu]

Problem

When an LLM response contains <at user_id="ou_xxx">Name</at> tags, the current buildPostContent() function wraps the entire text — including <at> tags — inside a single {tag: "md", text: "..."} element. Lark's post message API treats <at> tags embedded in markdown text as visual rendering only — it does NOT populate the mentions array or trigger im.message.receive_v1 events.

This blocks multi-bot collaboration scenarios where Bot A needs to @mention Bot B in group chats.

Verified behavior

Payload format	mentions array	Event triggered
{tag: "md", text: "...<at>..."} (current)	empty	No
{tag: "at", user_id: "..."} as standalone element	populated	Yes

Tested with Lark API POST /open-apis/im/v1/messages and verified via im.message.receive_v1 WebSocket events.

Change

buildPostContent() in src/messaging/outbound/deliver.ts now:

1. Unescapes \" → " (LLM output may contain escaped quotes)
2. Splits text on <at user_id="...">...</at> boundaries
3. Emits <at> matches as {tag: "at", user_id: "..."} elements
4. Emits surrounding text as {tag: "md", text: "..."} elements
5. Falls back to original behavior when no <at> tags are present

Impact

• Non-breaking: Messages without <at> tags behave identically to before
• Additive: Messages with <at> tags now produce real mentions
• Works with normalizeAtMentions() which already fixes common AI-generated <at> format errors

Use case

Multi-bot group chats where agents @mention each other to create discussion chains. Without this fix, the mention is rendered visually but the target bot never receives the event.

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