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>

AGENTS.md

@@ -36,10 +36,11 @@ the container lifecycle and the copying of skills and env vars into it.
 
 - Three kinds of doc, each with its own conventions in-folder; see
   `docs/README.md` for when to write which:
-  - **PRDs** (`docs/prds/`) — one feature per file, numbered
-    `NNNN-kebab.md`. A `Status:` line tracks lifecycle: Draft → Active
-    (shipped to `main`) → Superseded/Retargeted. Format in
-    `docs/prds/README.md`.
+  - **PRDs** (`docs/prds/`) — one feature per file. While a PR is open
+    the file is named `prd-new-<kebab>.md`; CI assigns a sequential
+    number on merge to `main` and renames it. A `Status:` line tracks
+    lifecycle: Draft → Active (shipped to `main`) →
+    Superseded/Retargeted. Format in `docs/prds/README.md`.
   - **Research notes** (`docs/research/`) — opinionated investigations;
     unnumbered kebab-case, freeform and verdict-first. See
     `docs/research/README.md`.