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

docs/design/multitab-pane.md

@@ -8,7 +8,7 @@ Date: 2026-05-06
 
 Some panes need to host multiple, dynamically-spawned views of the
 same kind. The first concrete case is the Claude pane: per
-[D-41](../../decisions/architecture.md#d-41-claude-panes-one-primary-per-repo-tmux-backed),
+[D-41](../../governance/decisions/architecture.md#d-41-claude-panes-one-primary-per-repo-tmux-backed),
 each repo has exactly one **primary** Claude pane plus zero or more
 **secondary** panes spawned at runtime. The user needs a way to:
 
@@ -31,7 +31,7 @@ diff/preview surfaces) can adopt it without reinventing tab strips.
 - Replacing `TabContribution`. Slot-host tabs are static-by-design;
   this widget is for inside-a-tab dynamism.
 - Window-level tab management (browser-style "tear off into a window").
-- Editor multi-buffer tabs. [D-48](../../decisions/architecture.md#d-48-chrome-budget-no-tabs-no-breadcrumbs-keyboard-first)
+- Editor multi-buffer tabs. [D-48](../../governance/decisions/architecture.md#d-48-chrome-budget-no-tabs-no-breadcrumbs-keyboard-first)
   rejected those; revisiting is a separate decision.
 
 ## API sketch