Create pf-validate-focus-management skill

[patternfly-jira-sync]

Audits focus management in complex interactions — focus traps, restoration after modal close, programmatic focus moves, live region announcements.

AC:

• Skill exists in a11y plugin

• Identifies focus trap issues in modals, drawers, and dropdowns

• Flags missing focus restoration on close

---

Jira Issue: PF-4176