docs: bound and shell-safe docker run callout in AGENTS.md

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

AGENTS.md

@@ -7,7 +7,7 @@
 
 ## Critical build callouts
 
-Read these two facts before you start a build phase. They already caused a
+Read these facts before you start a build phase. They already caused a
 confusing failure once.
 
 1. **Force IPv4 for every Gmail connection.** This applies to IMAP and SMTP.
@@ -15,6 +15,11 @@ confusing failure once.
    socket EOF over IPv6 from `.13`.
 2. **The source address header is `X-Pm-Original-To`** (added by PurelyMail).
    See FR-2 and §13.7. **Never use `Delivered-To`** — Gmail overwrites it.
+3. **Every `docker run` in a heartbeat must be bounded and shell-safe.** The image has
+   `ENTRYPOINT ["/usr/local/bin/email-agent"]`, so run shells with
+   `timeout 10 docker run --rm --entrypoint sh email-agent:dev -c '<cmd>'`. Never issue an
+   unbounded `docker run`; a bare `sh -c` becomes ARGUMENTS to the binary, starts the
+   server, and hangs the run.
 
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