feat(db): per-project work-item prefix in get_next_work_item_id

ORCH-6: get_next_work_item_id(repo, prefix="ET") numbers per (repo, prefix)
so orchestrator issues number ORCH-001 independently of the ET sequence.
Default prefix stays ET for backward compatibility.

src/db.py

@@ -79,21 +79,29 @@ def update_task_stage(task_id: int, stage: str):
     conn.close()
 
 
-def get_next_work_item_id(repo: str) -> str:
-    """Generate next work item ID (e.g., ET-003)."""
+def get_next_work_item_id(repo: str, prefix: str = "ET") -> str:
+    """Generate next work item ID (e.g., ET-003 / ORCH-001).
+
+    ORCH-6: numbering is per (repo, prefix). The prefix comes from the project
+    registry (proj.work_item_prefix), so orchestrator issues number ORCH-001,
+    ORCH-002 independently of the ET sequence in enduro-trails. Default prefix
+    stays "ET" for backward compatibility with existing callers.
+    """
     conn = get_db()
     row = conn.execute(
-        "SELECT work_item_id FROM tasks WHERE repo = ? AND work_item_id IS NOT NULL ORDER BY id DESC LIMIT 1",
-        (repo,),
+        "SELECT work_item_id FROM tasks "
+        "WHERE repo = ? AND work_item_id LIKE ? AND work_item_id IS NOT NULL "
+        "ORDER BY id DESC LIMIT 1",
+        (repo, f"{prefix}-%"),
     ).fetchone()
     conn.close()
 
     if row and row["work_item_id"]:
-        # Parse ET-003 -> 3, increment
-        prefix, num = row["work_item_id"].rsplit("-", 1)
+        # Parse <PREFIX>-003 -> 3, increment (keep the existing prefix).
+        existing_prefix, num = row["work_item_id"].rsplit("-", 1)
+        prefix = existing_prefix
         next_num = int(num) + 1
     else:
-        prefix = "ET"
         next_num = 1
 
     return f"{prefix}-{next_num:03d}"