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>

docs/claude-design/README.md

@@ -8,8 +8,8 @@
 > Update wireframes there, not here.
 >
 > **Why kept:** the design tokens under `tokens/` and `themes/` still
-> feed the runtime themes (per [D-43](../../decisions/architecture.md#d-43-design-handoff-adopt-token-palettes-reject-material-wrapper)
-> / [D-44](../../decisions/architecture.md#d-44-four-bundled-themes-clide-midnight-paper-terminal)).
+> feed the runtime themes (per [D-43](../../governance/decisions/architecture.md#d-43-design-handoff-adopt-token-palettes-reject-material-wrapper)
+> / [D-44](../../governance/decisions/architecture.md#d-44-four-bundled-themes-clide-midnight-paper-terminal)).
 > The HTMLs and PNGs are kept for historical context.
 >
 > **What changed since:** welcome screen has logo-with-wordmark and a