feat(webhook): derive work_item_id from Plane sequence_id (M-6)

src/webhooks/plane.py

@@ -154,8 +154,20 @@ async def handle_work_item_created(data: dict, project_id: str = ""):
         logger.info(f"QG-0 failed for {plane_id}: {errors}")
         return
 
-    # Generate work item ID
-    work_item_id = get_next_work_item_id(repo, proj.work_item_prefix)
+    # Generate work item ID.
+    # M-6: source of truth for the number is the Plane sequence_id. Fetch it by
+    # issue UUID; if Plane is unavailable, fall back to the DB increment so a
+    # Plane outage never blocks task creation (autonomy > exact numbering).
+    from ..plane_sync import fetch_issue_sequence_id
+    seq = fetch_issue_sequence_id(plane_id, plane_project_id)
+    if seq is not None:
+        work_item_id = f"{proj.work_item_prefix}-{seq:03d}"
+    else:
+        work_item_id = get_next_work_item_id(repo, proj.work_item_prefix)
+        logger.warning(
+            f"Plane sequence_id unavailable for {plane_id}, "
+            f"fell back to DB increment: {work_item_id}"
+        )
 
     # Create slug from name
     slug = re.sub(r"[^a-z0-9]+", "-", name.lower()).strip("-")[:30]