simplify skill scripts: expose find_repo_root, fix split string, simplify label

- z3db.py: rename _find_repo_root to find_repo_root (make public) and
  add require_repo_root helper that exits on failure; update docstring
  and internal caller; fix unnecessary string literal split in log()
- memory_safety.py: remove 13-line duplicate find_repo_root function;
  import and use require_repo_root from z3db instead
- static_analysis.py: simplify two-line label assignment in
  print_findings to a single expression using 'or'

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

.github/skills/static-analysis/scripts/static_analysis.py

@@ -176,9 +176,7 @@ def print_findings(findings: list):
         return
 
     for f in findings:
-        label = f["category"]
-        if f["type"]:
-            label = f["type"]
+        label = f["type"] or f["category"]
         print(f"[{label}] {f['file']}:{f['line']}: {f['description']}")
 
     print()