feat: log skip reason when detectors are skipped in scan

Previously, when a detector had skip=True (e.g., AlwaysSkip detector),
the harness silently continued with no indication to the user about why
the detector was not run.

Changes:
- Add skip_reason attribute to Detector base class
- Log the reason at INFO level when a detector is skipped
- Default reason is 'disabled via config' when skip_reason is not set

Fixes #1061

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Signed-off-by: Oxygen <1391083091@qq.com>

Author: Oxygen56

garak/detectors/base.py

@@ -42,6 +42,10 @@ class Detector(Configurable):
         "skip": False,
     }
 
+    skip_reason: str = None
+    """Human-readable reason why this detector was skipped. Set when ``skip`` is
+    True to help users understand why a detector did not run on a probe."""
+
     _run_params = {"seed"}
 
     def _set_description(self):

garak/harnesses/base.py

@@ -196,9 +196,16 @@ def run(self, model, probes, detectors, evaluator, announce_probe=True) -> None:
                 attempt_iterator = tqdm.tqdm(attempt_results, leave=False)
                 detector_probe_name = d.detectorname.replace("garak.detectors.", "")
                 attempt_iterator.set_description("detectors." + detector_probe_name)
+                if d.skip:
+                    reason = getattr(d, "skip_reason", None) or "disabled via config"
+                    logging.info(
+                        "skipping detector %s on probe %s: %s",
+                        detector_probe_name,
+                        probe.probename,
+                        reason,
+                    )
+                    continue
                 for attempt in attempt_iterator:
-                    if d.skip:
-                        continue
                     attempt.detector_results[detector_probe_name] = list(
                         d.detect(attempt)
                     )