update decisions/ → governance/ refs after D-21 migration

Stale path references from the move in 63195d1:
- CLAUDE.md: 8 D-record links, the layout tree, the open-questions
  pointer, all rewritten to governance/.
- docs/design/multitab-pane.md + docs/claude-design/README.md:
  cross-references updated.
- lib/clide.dart: doc-comment refs.
- lib/builtin/problems: user-facing message string.
- Makefile: decisions-validate target docstring.

Note: lib/builtin/decisions/ (the in-app decisions panel package)
keeps its name — it's a feature name, not a filesystem-path mirror.

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

Makefile

@@ -243,7 +243,7 @@ security: ## Dart advisory review.
 # -- pre-push gate --------------------------------------------------------
 
 .PHONY: decisions-validate
-decisions-validate: ## Parser dry-run over decisions/*.md.
+decisions-validate: ## Parser dry-run over governance/{decisions,questions,rejected}/*.md.
 	pql decisions validate
 
 .PHONY: push-check