docs(witness): update AutoNukeIfClean to reflect self-cleaning model

Updated comment to use "orphaned polecats" instead of "idle polecats".
With the self-cleaning model, polecats self-nuke on completion.
An orphan is from a crash, not a normal idle state.

Closes: gt-7l8y1

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

internal/witness/handlers.go

@@ -687,7 +687,9 @@ type NukePolecatResult struct {
 }
 
 // AutoNukeIfClean checks if a polecat is safe to nuke and nukes it if so.
-// This is used for idle polecats with no pending MR - they can be nuked immediately.
+// This is used for orphaned polecats (no hooked work, no pending MR).
+// With the self-cleaning model, polecats should self-nuke on completion.
+// An orphan is likely from a crash before gt done completed.
 // Returns whether the nuke was performed and any error.
 func AutoNukeIfClean(workDir, rigName, polecatName string) *NukePolecatResult {
 	result := &NukePolecatResult{}