Merge pull request #582: feat(tabs): pin tabs to a Focus section on the vertical rail

Conflicts resolved against #528:
- CLAUDE.md Owner tab layouts: #582's Focus sentences and #528's
  tabGroupId / spawnedTabsFollowParent sentences are both kept.
- i18n.js: both features' ZH_CN entries kept.
- tab-layout-browser.js: the section-model JSDoc documents #528's
  `options.parents` and #582's `focusRefs` return field.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

config/test-suites.ts

@@ -23,6 +23,7 @@ export const BROWSER_TEST_GLOBS = [
   'test/tab-activation.browser.test.ts',
   'test/tab-layout-editing.browser.test.ts',
   'test/tab-rail-search.browser.test.ts',
+  'test/tab-focus.browser.test.ts',
   'test/session-sidebar-ux.browser.test.ts',
   'test/session-options-responsive.browser.test.ts',
   'test/inline-rename.test.ts',