feat(v0.6): route lease-freshness callsites through IsStale

InspectLease, CleanupStaleLease, runActiveLeaseCheck, and statusAt now
use the PID-aware IsStale predicate. A dead local owner PID makes a lease
stale immediately; SessionStatus / list / info reflect this with no
display-code change.

Corrects three cmd-package session-display test fixtures that built
"active" leases with the local hostname but synthetic PIDs — now that
freshness is PID-aware, an active session must be owned by a live
process, so the fixtures use os.Getpid().

internal/session/lease.go

@@ -189,7 +189,7 @@ func CleanupStaleLease(path string, staleAfter time.Duration) (*Lease, error) {
 		}
 		return nil, nil
 	}
-	if IsFresh(&l, time.Now(), staleAfter) {
+	if !IsStale(&l, time.Now(), staleAfter) {
 		return nil, nil
 	}
 	if rmErr := os.Remove(path); rmErr != nil && !errors.Is(rmErr, os.ErrNotExist) {