fix: log skipped invalid gitignore patterns to stderr

Silent pass on re.error hid invalid .gitignore patterns from users,
making it impossible to diagnose unexpected file-exclusion behavior.
Now logs a warning to stderr so users can see which patterns were
skipped and why.

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

understand-anything-plugin/skills/understand-domain/extract-domain-context.py

@@ -133,8 +133,8 @@ def parse_gitignore(project_root: Path) -> list[re.Pattern[str]]:
             regex = regex.rstrip("/") + "(/|$)"
         try:
             patterns.append(re.compile(regex))
-        except re.error:
-            pass
+        except re.error as e:
+            print(f"Warning: skipping invalid gitignore pattern '{line}': {e}", file=sys.stderr)
     return patterns