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_test.go

@@ -403,7 +403,8 @@ func TestStaleCommandInit(t *testing.T) {
 	if flags.Lookup("limit") == nil {
 		t.Error("staleCmd should have --limit flag")
 	}
-	if flags.Lookup("json") == nil {
-		t.Error("staleCmd should have --json flag")
+	// --json is inherited from rootCmd as a persistent flag
+	if staleCmd.InheritedFlags().Lookup("json") == nil {
+		t.Error("staleCmd should inherit --json flag from rootCmd")
 	}
 }