docs: pin cwd meaning in attribution footer

Mirror of command-center clarification: first dogfooding round stamped the
build worktree instead of the session's own working directory. Rule text
now says which one, and to note the build worktree in the body.

AGENTS.md

@@ -78,7 +78,7 @@ the same regime.
 cwd: <working directory> · session: <omp session id>
 ```
 
-Session id from `/status`. Machine-made artifacts stay attributable to the box and session that spawned them.
+Session id from `/status`; `cwd` is that session's own working directory — not the build checkout, when they differ (note the build worktree in the body instead). Machine-made artifacts stay attributable to the box and session that spawned them.
 
 ## Deployment