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/README.md

@@ -394,8 +394,11 @@ When `KB_BADGE_PUBSUB_REDIS_URL` is not set, the dashboard uses an in-memory ada
 
 ### Configuration
 - `GET /api/config` - Server configuration
-- `GET /api/settings` - User settings
-- `PUT /api/settings` - Update settings
+- `GET /api/settings` - Merged settings (project overrides global)
+- `PUT /api/settings` - Update project-level settings (rejects global-only fields)
+- `GET /api/settings/global` - Global user settings (~/.pi/kb/settings.json)
+- `PUT /api/settings/global` - Update global user settings
+- `GET /api/settings/scopes` - Settings separated by scope: { global, project }
 - `GET /api/models` - Available AI models
 - `GET /api/auth/status` - OAuth provider status
 - `POST /api/auth/login` - Initiate OAuth login