ref(onboarding): Use LayoutGroup for scmConnect footer position animation

[jaydgoss]

Summary

• Use LayoutGroup + layout="position" to animate the scmConnect footer position when content above changes height (e.g. benefits card appearing after repo selection)
• Restructure content sections as direct motion siblings of the footer within the LayoutGroup so framer-motion can detect layout changes
• Remove ScmStepContent wrapper in favor of inline MotionStack with width/maxWidth props
• Use MotionStack wrapper around ScmStepFooter instead of motion.create(ScmStepFooter), since layout="position" needs to be on the actual DOM element

Stacked on VDY-24 platform features polish PR.

Test plan

• Connect an SCM integration, select a repo, verify footer slides smoothly when benefits card appears
• Verify no layout jump when switching between connected/not-connected states

[linear-code]

VDY-24 SCM platform-features step: UI polish and analytics pass

[jaydgoss]

TODO: The loading and error early returns in scmConnect.tsx (lines 63-78) render outside the step layout (no header, no LayoutGroup). They should be moved into the main return inside the LayoutGroup section (lines 89-121) so the step header stays visible and the layout is consistent across all states.

[jaydgoss]

Commit cherry-picked into #111478. This PR is no longer needed.