[codex] Add container-local focus cycling

[Knapsack44]

Summary

Add focus container-next and focus container-prev for container-local sibling cycling.

Behavior

• Cycles only among direct sibling windows in the focused window's parent container.
• Wraps within that container.
• No-ops when there is only one focusable child.
• Keeps existing workspace-wide focus behavior unchanged.

Validation

• swift test --filter FocusCommandTest
• Live test on my system:
  • moved two windows into a workspace container
  • verified focus container-next toggled focus between them
  • verified focus container-prev toggled back