[codex] Refactor skills workspace scroll ownership

[SI-RUI-ZHANG]

Summary

Refactors the Skills workspace so scroll ownership is explicit and singular.

• adds a shared pane scaffold for managed and unmanaged tabs
• makes tab scroll persistence pane-scroller-only instead of switching to window.scrollY
• moves the Skills route onto a contained shell so the document no longer scrolls behind the workspace
• removes the old responsive fallback that reverted the Skills page to normal page scrolling
• updates browser coverage to assert scroll containment and scroll restoration

Root Cause

The list scroller already existed, but the outer Skills route shell still allowed document-level overflow. On narrower widths, the workspace also had a separate breakpoint path that intentionally switched back to page scroll. That left two competing scroll models and allowed the whole pane to move instead of keeping header/search chrome fixed.

Validation

• bash scripts/ci_validate.sh
• npm test
• npm run test:e2e

Impact

The Skills page now behaves consistently across both tabs and across widths: header/search chrome stays fixed within the workspace and the cards list owns scrolling.