fix(plugin): don't record false success for manual plugin runs

`gt plugin run` was recording ResultSuccess even though it only prints
instructions without executing them. This poisoned the cooldown gate,
blocking actual executions for 24h.

Changes:
- Record manual runs as ResultSkipped instead of ResultSuccess
- Add CountSuccessfulRunsSince() to only count successful runs for gate
- Gate check now uses CountSuccessfulRunsSince() so skipped/failed runs
  don't block future executions

Fixes: hq-2dis4c

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

internal/plugin/recording.go

@@ -213,3 +213,19 @@ func (r *Recorder) CountRunsSince(pluginName string, since string) (int, error)
 	}
 	return len(runs), nil
 }
+
+// CountSuccessfulRunsSince returns the count of successful runs for a plugin since the given duration.
+// Only successful runs count for cooldown gate evaluation - skipped/failed runs don't reset the cooldown.
+func (r *Recorder) CountSuccessfulRunsSince(pluginName string, since string) (int, error) {
+	runs, err := r.GetRunsSince(pluginName, since)
+	if err != nil {
+		return 0, err
+	}
+	count := 0
+	for _, run := range runs {
+		if run.Result == ResultSuccess {
+			count++
+		}
+	}
+	return count, nil
+}