feat(KB-185): add global/project settings hierarchy with scope-aware UI

- Add GlobalSettingsStore with file-based storage in ~/.pi/kb/settings.json
- Implement two-tier hierarchy: global (user prefs) and project (.kb/config.json)
- Update TaskStore to merge settings with project overriding global
- Add API endpoints: GET/PUT /api/settings/global and GET /api/settings/scopes
- Add scope indicators (🌐 global, 📁 project) to settings modal sidebar
- Implement scope-aware save: only save active scope, reject global-only fields
- Add frontend API functions for global settings operations
- Add tests for GlobalSettingsStore, settings routes, and SettingsModal scopes
- Create changeset for the new settings hierarchy feature
- Update AGENTS.md with settings hierarchy documentation

packages/dashboard/app/styles.css

@@ -1170,6 +1170,33 @@ body {
   margin-bottom: 4px;
 }
 
+/* Scope indicators in sidebar nav items */
+.settings-scope-icon {
+  margin-right: 6px;
+  font-size: 12px;
+  line-height: 1;
+}
+
+/* Scope banner above section content */
+.settings-scope-banner {
+  display: flex;
+  align-items: center;
+  gap: 8px;
+  padding: 8px 20px;
+  margin: 0 0 4px 0;
+  font-size: 12px;
+  border-radius: var(--radius);
+  color: var(--text-muted);
+}
+.settings-scope-global {
+  background: rgba(59, 130, 246, 0.08);
+  border-left: 3px solid rgba(59, 130, 246, 0.4);
+}
+.settings-scope-project {
+  background: rgba(34, 197, 94, 0.08);
+  border-left: 3px solid rgba(34, 197, 94, 0.4);
+}
+
 /* === Auth Provider Cards === */
 .auth-provider-row {
   display: flex;