fix(json): audit and standardize JSON output across commands (bd-au0.7)

Audit findings:
- All commands properly respect --json flag for success output
- Added outputJSONError() helper for consistent JSON error output
- Removed redundant local --json flag from stale.go (inherited from rootCmd)
- Fixed stale_test.go to check InheritedFlags() instead of local Flags()

JSON output patterns verified across:
- Query commands: ready, blocked, stale, count, stats, status
- Dep commands: dep add/remove/tree/cycles
- Label commands: label add/remove/list/list-all
- Comment commands: comments add/list
- Epic commands: epic status/close-eligible

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

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

cmd/bd/stale.go

@@ -110,6 +110,6 @@ func init() {
 	staleCmd.Flags().IntP("days", "d", 30, "Issues not updated in this many days")
 	staleCmd.Flags().StringP("status", "s", "", "Filter by status (open|in_progress|blocked|deferred)")
 	staleCmd.Flags().IntP("limit", "n", 50, "Maximum issues to show")
-	staleCmd.Flags().BoolVar(&jsonOutput, "json", false, "Output JSON format")
+	// Note: --json flag is defined as a persistent flag in main.go, not here
 	rootCmd.AddCommand(staleCmd)
 }