Show expandable active goals in the chat progress rail

[hamzamerzic]

Summary

• show the active /goal objective in the compact progress rail above the composer
• let truncated goals expand and collapse by click or keyboard while short goals remain quiet
• keep the goal visible across mid-turn steering and cached chat remounts
• reuse the same rail for build milestones, then retire the goal when the run settles or stops
• announce the active objective through the chat's existing accessible status

Testing

• npm test
• npm run build
• focused goal, build-rail, and footer lifecycle tests
• rendered expand-and-collapse verification in the live shell

[hamzamerzic]

Superseded by the reviewed visual integration in #295, which preserves this change and its authorship. The integration is now moving through the merge queue.