refactor: add usecase layer for staging operations

[mpyw]

Summary

• Add internal/usecase/staging/ package with structured input/output types
• Create usecases for: status, apply, add, edit, delete, reset, diff
• Foundation for supporting both CLI and future GUI (Wails) interfaces

This is Step 2 of the usecase layer architecture refactoring plan.

Note: The existing runners are not yet modified to use these usecases. That integration will be done in a subsequent PR.

Data Flow (Target)

Before:
  CLI Command → Runner → io.Writer (formatted text)

After:
  CLI Command → UseCase → Structured Data → CLI formats to text
  GUI         → UseCase → Structured Data → GUI renders to UI

Test plan

• Build passes
• Lint passes
• Integration with runners (future PR)

🤖 Generated with Claude Code

[codecov]

Codecov Report

❌ Patch coverage is 89.22156% with 36 lines in your changes missing coverage. Please review.
✅ Project coverage is 88.84%. Comparing base (4da01a2) to head (ff46ddd).

Files with missing lines	Patch %	Lines
internal/usecase/staging/reset.go	84.37%	5 Missing and 5 partials ⚠️
internal/usecase/staging/diff.go	90.24%	6 Missing and 2 partials ⚠️
internal/usecase/staging/edit.go	81.39%	4 Missing and 4 partials ⚠️
internal/usecase/staging/add.go	90.62%	2 Missing and 1 partial ⚠️
internal/usecase/staging/status.go	91.42%	2 Missing and 1 partial ⚠️
internal/usecase/staging/apply.go	95.91%	1 Missing and 1 partial ⚠️
internal/usecase/staging/delete.go	93.10%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@                    Coverage Diff                     @@
##           refactor/usecase-layer       #3      +/-   ##
==========================================================
+ Coverage                   88.81%   88.84%   +0.03%     
==========================================================
  Files                          58       65       +7     
  Lines                        4059     4393     +334     
==========================================================
+ Hits                         3605     3903     +298     
- Misses                        304      325      +21     
- Partials                      150      165      +15     

Flag	Coverage Δ
e2e	66.97% <ø> (ø)
unittests	78.96% <89.22%> (+0.84%)	⬆️

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[mpyw]

レビュー結果

修正済み

• ✅ インターフェース移動: DeleteStrategy, VersionFetcher → staging package
• ✅ メソッドリネーム: GetStagedValue → Draft, GetCurrentValue → Baseline
• ✅ apply.go: name filterのチェック順序を修正（空チェックより前に）
• ✅ テスト追加（カバレッジ 91.9%）

確認事項

• エラーラッピングは現状で一貫性あり（修正不要）
• _ context.Context の使用は将来使う時に変更でOK

残課題（別PR予定）

• staging.Store のインターフェース化（テストでモック可能にする）
  • 現在のカバレッジ91.9%の残り8%はStore操作のエラーパス
  • 実ファイルシステムを使うためモック不可