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

src/plane_sync.py

@@ -71,6 +71,24 @@ STAGE_TO_STATE = {
 }
 
 
+def fetch_issue_sequence_id(issue_id: str, project_id: str) -> int | None:
+    """M-6: GET the Plane issue by UUID and return its sequence_id (the
+    authoritative per-project number), or None if unavailable.
+
+    Returns None on network error, non-2xx, or a missing field - never raises,
+    so the webhook handler can fall back to DB increment and stay autonomous.
+    """
+    url = f"{PLANE_BASE}/workspaces/{WORKSPACE}/projects/{project_id}/issues/{issue_id}/"
+    try:
+        resp = httpx.get(url, headers=PLANE_HEADERS, timeout=10)
+        resp.raise_for_status()
+        seq = resp.json().get("sequence_id")
+        return int(seq) if seq is not None else None
+    except Exception as e:
+        logger.warning(f"fetch_issue_sequence_id failed for {issue_id}: {e}")
+        return None
+
+
 def find_issue_id(work_item_id: str, project_id: str = None) -> str | None:
     """Find Plane issue UUID by work_item_id (e.g. 'ET-002')."""
     project_id = _resolve_project_id(work_item_id, project_id)

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]