fix: add_label falls back gracefully on sub-sheet schematics

In hierarchical KiCad designs, sub-sheets (.kicad_sch files referenced
via (sheet ...) blocks) do not have a (sheet_instances) section — that
only appears in the top-level schematic.

WireManager.add_label was returning False with a logged error whenever
no (sheet_instances) marker was found, making it impossible to add any
net label (local, global, or hierarchical) to a sub-sheet.

Fix: fall back to inserting before the final item of the s-expression
list, which is equivalent to appending before the closing ')' of the
outer (kicad_sch ...) block.

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

python/commands/wire_manager.py

@@ -316,8 +316,9 @@ class WireManager:
                     break
 
             if sheet_instances_index is None:
-                logger.error("No sheet_instances section found in schematic")
-                return False
+                # Sub-sheets in hierarchical designs don't have (sheet_instances).
+                # Fall back to appending before the final closing paren of (kicad_sch ...).
+                sheet_instances_index = len(sch_data)
 
             # Insert label
             sch_data.insert(sheet_instances_index, label_sexp)