docs(bootstrap): document ae_send_message recipient names and inbox gotcha

Recipient names (frontend, backend) don't match inbox directory names
(frontend_svelte, backend_fastapi) — this tripped up agent messaging.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

documentation/BOOTSTRAP__AI_Agent_Quickstart.md

@@ -422,7 +422,40 @@ documentation/
 
 ---
 
-## 14. Reading Order for Deeper Dives
+## 14. Agent Messaging (ae_send_message / ae_inbox)
+
+Use MCP tools to coordinate with other agents — do not write directly to the inbox directories.
+
+```python
+# Send a message
+ae_send_message(recipient='frontend', sender='backend_fastapi', content='...')
+
+# Check your inbox
+ae_inbox(command='list', status='unread')
+```
+
+### Valid `ae_send_message` recipient names
+
+| Recipient name | Inbox directory | Agent |
+|---|---|---|
+| `frontend` | `~/agents_sync/inbox/frontend_svelte/` | SvelteKit frontend agent |
+| `backend` | `~/agents_sync/inbox/backend_fastapi/` | FastAPI backend agent (this agent) |
+| `mcp_agent` | `~/agents_sync/inbox/mcp_agent/` | MCP/orchestration agent |
+| `scott_wks` | `~/agents_sync/inbox/scott_wks/` | Scott's workstation |
+| `scott_lpt` | `~/agents_sync/inbox/scott_lpt/` | Scott's laptop |
+| `gemini_cli` | `~/agents_sync/inbox/gemini_cli/` | Gemini CLI agent |
+
+> **Gotcha:** The inbox directory names (`frontend_svelte`, `backend_fastapi`) do NOT match the
+> `ae_send_message` recipient names (`frontend`, `backend`). Always use the recipient names above —
+> using the directory name will produce an `invalid choice` error.
+
+### Checking your own inbox
+
+The `ae_inbox` MCP tool reads from the `backend_fastapi` inbox by default when invoked in this project directory. Use `status='unread'` to see new messages, `status='all'` to see everything.
+
+---
+
+## 15. Reading Order for Deeper Dives
 
 | What you need | Read |
 |---|---|