chore: update pr changelog naming

Author: piotr-iohk

.agents/commands/pr.md

@@ -227,12 +227,12 @@ gh pr create --base $base --title "..." --body "..." [--draft]
   - Instruct the user to drag-and-drop each media file into the Preview section via the GitHub web UI
   - Note: GitHub does not support programmatic media upload to PR bodies
 
-### 8b. Backfill Changelog PR Number
+### 8b. Changelog Fragments
 
-If the PR was created (not dry run) and `CHANGELOG.md` was modified in the branch (`git diff $base...HEAD --name-only | grep CHANGELOG.md`):
-- Read `CHANGELOG.md` and scan `## [Unreleased]` entries for lines missing a `#NUMBER` suffix
-- Append the actual PR number to those entries, e.g. `- Add foo` becomes `- Add foo #123`
-- If any entries were updated, create a new commit with message `chore: backfill changelog pr number` and push
+If the PR is user-facing, verify the branch adds exactly one changelog fragment under `changelog.d/next/` or `changelog.d/hotfix/`.
+Do not edit `CHANGELOG.md` in normal PRs.
+If the PR was created (not dry run), rename any new changelog fragment whose filename does not start with the actual PR number to `<PR_NUMBER>.<category>.md`, preserving the category (`added`, `changed`, `deprecated`, `removed`, `fixed`, or `security`).
+If any fragment was renamed, create a follow-up commit with message `chore: rename changelog fragment` and push it.
 
 ### 9. Output Summary