docs: put the closing keyword in the commit subject

GitHub matches a closing keyword anywhere in the message, so the previous
"on its own line in the body" rule was invented. Use the parenthesised subject
form instead: "fix: ... (closes #1040)".

AGENTS.md

@@ -96,7 +96,23 @@ release the same day. **Master is continuously released** — a PR must be
 release-ready exactly as merged; there is no stabilization window for follow-up
 fixes.
 
-## Code style
+## Commit messages
+
+A commit that resolves an issue must close it, with a GitHub closing keyword in
+parentheses at the end of the subject line:
+
+```
+fix: stop metadata probes from writing playlist sidecar files (closes #1040)
+```
+
+Because master is the default branch and is released on every push, the issue
+closes at the moment the fix ships, and keeps a permanent link to the commit that
+fixed it. A bare `(#1040)` is only a reference — and reads as a pull-request
+number — so it does not count; the keyword is what closes the issue.
+
+Auto-closing leaves only a commit stub on the issue, which is not an answer to
+whoever reported it. Post an explanatory comment as well: what the cause was, what
+changed, and anything the reporter needs to do differently.
 
 Follow `.editorconfig`:
 - Python: 4-space indent