Add comprehensive Focus management reference

[AvdLee]

Summary

• Add references/focus-patterns.md covering the full SwiftUI Focus API surface: @FocusState, .focusable(), FocusInteractions, @FocusedValue/@FocusedBinding/@FocusedObject, .defaultFocus, focus scope/sections, focus effects, search focus, and common pitfalls
• Update SKILL.md with Topic Router entry, References entry, and two Correctness Checklist items for focus
• Replace inline @FocusState section in state-management.md with a cross-reference to the new dedicated file

Fixes #48

Motivation

Issue #48 requested best practices around Focus, @FocusState, and related APIs. The existing skill had only a minimal 35-line @FocusState section in state-management.md covering basic Bool/enum usage. Apple's Focus API surface includes 20+ APIs across 8 categories, most of which were undocumented in the skill.

The new reference file covers the specific gotcha from the issue (redundant @FocusState writes in tap gesture handlers on .focusable() views causing focus to briefly appear then disappear) as a documented common pitfall.

Test plan

• Verify focus-patterns.md renders correctly and all code examples compile
• Verify cross-reference from state-management.md points to the correct file
• Verify Topic Router and References in SKILL.md include the new entry
• Verify Correctness Checklist items are present

[Copilot]

Pull request overview

Adds a new SwiftUI focus-management reference to the swiftui-expert-skill docs, and wires it into the skill’s router/references so agents can discover and use it while keeping existing docs lean.

Changes:

• Add references/focus-patterns.md covering SwiftUI Focus APIs and common pitfalls.
• Update SKILL.md to route “Focus management” tasks to the new reference and add focus-related checklist items.
• Replace the inline @FocusState section in references/state-management.md with a cross-reference to the new focus reference (and update README structure list).

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 1 comment.

File	Description
swiftui-expert-skill/SKILL.md	Adds Focus management to the Topic Router, references list, and correctness checklist.
swiftui-expert-skill/references/state-management.md	Replaces the inline @FocusState guidance with a pointer to the new focus reference.
swiftui-expert-skill/references/focus-patterns.md	New comprehensive Focus API reference and pitfalls section.
README.md	Adds the new reference file to the documented skill structure list.