fix: remove unused isPow2 (golangci-lint unused) breaking CI on main

Leftover from the stall-escalation work: the milestone-map approach
replaced the power-of-two cadence check but the helper stayed. Local go
vet does not flag unused funcs; CI's golangci-lint does.

Author: jkyberneees

internal/loop/loop.go

@@ -3881,11 +3881,6 @@ func isBGPollTool(name string) bool {
 	return name == "bg_status" || name == "bg_output"
 }
 
-// isPow2 reports whether n is a positive power of two.
-func isPow2(n int) bool {
-	return n > 0 && n&(n-1) == 0
-}
-
 // againSuffix marks an escalated (repeat) stall hint so the model can tell
 // it apart from the first one — the loop is persistent, not self-correcting.
 func againSuffix(repeat bool) string {