ci(prd): add prd-new placeholder convention and numbering workflow

Implements #213: PRDs use prd-new-<slug>.md while a PR is open; a
post-merge workflow on main assigns sequential numbers and renames the
file. A required PR check blocks prd-new-*.md from landing on main
without going through the workflow.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

docs/prds/README.md

@@ -7,9 +7,12 @@ document vs. a research note or a decision record).
 
 ## Naming and numbering
 
-`NNNN-kebab-title.md`, zero-padded and sequential (`0024-…`, `0025-…`).
-Numbers are never reused; gaps are fine (there is no 0005). The number
-is assigned at creation and stays fixed for the life of the doc.
+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.
+
+Once numbered, the filename stays fixed for the life of the doc.
 
 ## Status
 
@@ -23,7 +26,7 @@ The `Status:` line near the top tracks the PRD's lifecycle:
 ## Format
 
 ```markdown
-# PRD NNNN: <short title>
+# PRD prd-new: <short title>    ← placeholder; CI fills in the number on merge
 
 - **Status:** Draft
 - **Author:** <who>