ci(prd): require manual numbering before merge

docs/prds/README.md

@@ -7,10 +7,13 @@ document vs. a research note or a decision record).
 
 ## Naming and numbering
 
-New PRDs use a `prd-new-<kebab-title>.md` placeholder name while the PR
-is open. On merge to `main` a CI workflow assigns the next sequential
-number (`0024-…`, `0025-…`), renames the file, and updates the title
-header. Numbers are never reused; gaps are fine.
+New PRDs may use a `prd-new-<kebab-title>.md` placeholder name while the
+design is being drafted. Before merge, assign the next sequential number
+after the highest-numbered PRD on `main`, rename the file to
+`NNNN-<kebab-title>.md`, and update the title header. CI blocks merging
+while any `prd-new-*.md` placeholder remains. If concurrent PRs select the
+same number, the later PR must take the next available number before it
+merges. Numbers are never reused; gaps are fine.
 
 Once numbered, the filename stays fixed for the life of the doc.
 
@@ -26,7 +29,7 @@ The `Status:` line near the top tracks the PRD's lifecycle:
 ## Format
 
 ```markdown
-# PRD prd-new: <short title>    ← placeholder; CI fills in the number on merge
+# PRD prd-new: <short title>    ← replace with the final number before merge
 
 - **Status:** Draft
 - **Author:** <who>