fix: 3 syntax errors in perf_bottleneck_finder.py (closes #211)

- Line 116: escaped quotes in regex pattern for file write detection
- Line 509: split newline join return statement
- Line 523: moved global declaration before first use

All 25 test_perf_bottleneck_finder tests pass.
python3 -m py_compile succeeds.

scripts/perf_bottleneck_finder.py

@@ -113,7 +113,7 @@ def find_slow_tests_by_scan(repo_path: str) -> List[Bottleneck]:
         (r"time\.sleep\((\d+(?:\.\d+)?)\)", "Contains time.sleep() — consider using mock or async wait"),
         (r"subprocess\.run\(.*timeout=(\d+)", "Subprocess with timeout — may block test"),
         (r"requests\.(get|post|put|delete)\(", "Real HTTP call — mock with responses or httpretty"),
-        (r"open\([^)]*['"]w['"]", "File I/O in test — use tmp_path fixture"),
+        (r"open\([^)]*['\"]w['\"]\)", "File I/O in test — use tmp_path fixture"),
     ]
 
     for root, dirs, files in os.walk(repo_path):
@@ -506,13 +506,13 @@ def format_markdown(report: PerfReport) -> str:
                 lines.append(f"- {icon} {b.name}{loc} — ~{b.duration_s:.1f}s — {b.recommendation}")
             lines.append(f"")
 
-    return "
-".join(lines)
+    return "\n".join(lines)
 
 
 # ── Main ───────────────────────────────────────────────────────────
 
 def main():
+    global SLOW_TEST_THRESHOLD_S
     parser = argparse.ArgumentParser(description="Performance Bottleneck Finder")
     parser.add_argument("--repo", default=".", help="Path to repository to analyze")
     parser.add_argument("--json", action="store_true", help="Output as JSON")
@@ -521,7 +521,6 @@ def main():
                         help="Slow test threshold in seconds")
     args = parser.parse_args()
 
-    global SLOW_TEST_THRESHOLD_S
     SLOW_TEST_THRESHOLD_S = args.threshold
 
     if not os.path.isdir(args.repo):