Add tooltips to worktree new-thread buttons

[charpeni]

Summary

• add a descriptive tooltip to the icon-only “Create new thread in this worktree” action
• show the tooltip in both the prompt footer and secondary-panel environment row
• preserve the existing accessible label and click behavior
• add keyboard-focus regression coverage for both locations

Testing

• pnpm exec turbo run test --filter=@bb/app --force -- src/components/promptbox/ThreadEnvironmentSummary.test.tsx src/components/secondary-panel/ThreadMetadataContent.test.tsx
• pnpm exec turbo run typecheck --filter=@bb/app
• git diff --check

All 7 focused tests passed and the @bb/app typecheck completed successfully.