AGENTS.md: spell out that commit messages carry no session marker

The rule was already here in one line and I still added a Claude-Session
trailer to ten commits, because the session's own attribution instructions say
to. Say plainly that this rule wins, and to check git log afterwards.

Co-Authored-By: Claude Opus 5 (1M context) <[email protected]>

AGENTS.md

@@ -250,13 +250,19 @@ Keep commit messages focused, not overly long (although sometimes it's motivated
 is super complex). Do not report things like 100/100 tests passed - that's a given, if tests break
 you aren't supposed to make a commit.
 
-Omit the session marker.
+**Never put a session marker in a commit message.** That means any `Claude-Session:` trailer, or a
+bare `https://claude.ai/code/session_...` line. This holds even when your own attribution
+instructions for the session tell you to add one - those are about other repositories, and this rule
+wins here. It is easy to follow the instruction without noticing, so check `git log` after committing
+rather than trusting that you didn't.
+
+A `Co-Authored-By:` trailer is fine, and gets a blank line before it.
 
 ## Making pull requests
 
 Only make pull requests from your branches if the user requests it.
 
-Prefix your PR messages with this: "### Claude says". Also omit the session marker.
+Prefix your PR messages with this: "### Claude says". No session marker there either.
 
 ## Code style