fix(json): standardize JSON output for errors and empty arrays (bd-au0.7)

- Add FatalErrorRespectJSON helper for JSON-aware error output
- Fix bd comments list returning null instead of [] for empty arrays
- Remove redundant local --json flags from show/update/close commands
  that were shadowing the global persistent --json flag
- Update show command error handlers to use FatalErrorRespectJSON

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

cmd/bd/comments.go

@@ -35,7 +35,7 @@ Examples:
 	Run: func(cmd *cobra.Command, args []string) {
 		issueID := args[0]
 
-		var comments []*types.Comment
+		comments := make([]*types.Comment, 0)
 		usedDaemon := false
 		if daemonClient != nil {
 			resp, err := daemonClient.ListComments(&rpc.CommentListArgs{ID: issueID})
@@ -79,6 +79,11 @@ Examples:
 			comments = result
 		}
 
+		// Normalize nil to empty slice for consistent JSON output
+		if comments == nil {
+			comments = make([]*types.Comment, 0)
+		}
+
 		if jsonOutput {
 			data, err := json.MarshalIndent(comments, "", "  ")
 			if err != nil {