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/errors.go

@@ -1,6 +1,7 @@
 package main
 
 import (
+	"encoding/json"
 	"fmt"
 	"os"
 )
@@ -23,6 +24,28 @@ func FatalError(format string, args ...interface{}) {
 	os.Exit(1)
 }
 
+// FatalErrorRespectJSON writes an error message and exits with code 1.
+// If --json flag is set, outputs structured JSON to stdout.
+// Otherwise, outputs plain text to stderr.
+//
+// Use this for errors in commands that support --json output.
+//
+// Example:
+//
+//	if err := store.GetIssue(ctx, id); err != nil {
+//	    FatalErrorRespectJSON("%v", err)
+//	}
+func FatalErrorRespectJSON(format string, args ...interface{}) {
+	msg := fmt.Sprintf(format, args...)
+	if jsonOutput {
+		data, _ := json.MarshalIndent(map[string]string{"error": msg}, "", "  ")
+		fmt.Println(string(data))
+	} else {
+		fmt.Fprintf(os.Stderr, "Error: %s\n", msg)
+	}
+	os.Exit(1)
+}
+
 // FatalErrorWithHint writes an error message with a hint to stderr and exits.
 // Use this when you can provide an actionable suggestion to fix the error.
 //