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/main.go

@@ -296,7 +296,8 @@ var rootCmd = &cobra.Command{
 					}
 
 					isNoDbMode := false
-					if configData, err := os.ReadFile(configPath); err == nil {
+					// configPath is safe: constructed from filepath.Join(beadsDir, hardcoded name)
+					if configData, err := os.ReadFile(configPath); err == nil { //nolint:gosec
 						isNoDbMode = strings.Contains(string(configData), "no-db: true")
 					}