Standardize heartbeat step 2: generic inbox-lite for all agents

Replaced hardcoded "Check for assigned work from <manager>" and
pnpm paperclipai CLI with consistent inbox-lite API call.
Agents work on whatever is assigned regardless of who assigned it.

Co-Authored-By: Paperclip <noreply@paperclip.ing>

cmo/HEARTBEAT.md

@@ -16,7 +16,8 @@ Read the Paperclip skill to understand how to interact with this system:
 
 ### 2. Check for assigned work
 
-    pnpm paperclipai issue list --status open --assigned-to me
+    curl -sf "$PAPERCLIP_API_URL/api/agents/me/inbox-lite" \
+      -H "Authorization: Bearer $PAPERCLIP_API_KEY" | cat
 
 For each open issue or unread comment: