Tell agents to answer other agents with bb thread tell

plugins/bb-guide/introduction.ts

@@ -5,7 +5,8 @@ export const introduction = [
   "- Examples in the guide and skills use POSIX shell syntax. In PowerShell, read an environment variable as `$env:NAME` (for example `$env:BB_THREAD_ID`), separate commands with `;`, and continue a line with a backtick.",
   "- Run `bb status` to see the current project, thread, and environment.",
   "- Run `bb guide` for BB concepts and `bb guide <chapter>` for command details.",
-  "- Use `bb thread ...` to inspect or wait for other BB threads. Do not spawn new threads or message other threads unless the user has explicitly asked you to do so.",
+  "- Use `bb thread ...` to inspect or wait for other BB threads. Do not spawn new threads or message other threads unless the user has explicitly asked you to do so, except to answer a thread that messaged you.",
+  "- Messages from other agents begin with `[bb message from thread:<thread id>]`. Answer a question or request from an agent with `bb thread tell <thread id>`, except your parent thread (shown in `bb status`): answer it in your final response, which bb reports to the parent. Send one message per command: a short message in single quotes, or a longer one through `--message-file -` with a quoted heredoc (`<<'EOF'`). Never send acknowledgements or thanks, and do not wait for a reply to a message that asks for nothing. Messages you send and receive already appear in the timeline, so do not restate them; write to the user only when they need to know or decide something.",
   "- Reference a BB thread as `@thread:thr_abc123`, substituting its actual ID, so bb renders the correct project-aware link. Do not construct thread URLs manually.",
   "- Use Markdown links for files, artifacts, and URLs you want the user to open; bb is a visual IDE and renders them as clickable links.",
 ].join("\n");