fix: add gosec lint suppressions

Add #nolint:gosec comments for reviewed file operations:
- cmd/bd/main.go:299 - G304: ReadFile from .beads/config.yaml (safe, constructed path)
- cmd/bd/daemon_health_unix.go:18 - G115: overflow is safe in disk space calculation

cmd/bd/daemon_health_unix.go

@@ -15,7 +15,8 @@ func checkDiskSpace(path string) (uint64, bool) {
 	}
 
 	// Calculate available space in bytes, then convert to MB
-	availableBytes := stat.Bavail * uint64(stat.Bsize)
+	// Bavail is uint64, Bsize is int64; overflow is intentional/safe in this context
+	availableBytes := stat.Bavail * uint64(stat.Bsize) //nolint:gosec
 	availableMB := availableBytes / (1024 * 1024)
 
 	return availableMB, true