fix: CodeRabbitレビュー指摘の修正（PR #14）

[myuron]

PR #14（Bubble Tea移行）に対する CodeRabbit のレビュー指摘 9 件を修正します。マージ済みのため別PRとして起票しました。

修正内容

#	重要度	ファイル	内容
1	🟠 Major	model.go / keybinding.go	? 後方検索の入口を追加（searchBackward() ＋ ? マップ）
2	🟠 Major	keybinding.go	pasteOverwrite で同一パス上書きをガード
3	🟡 Minor	keybinding.go	検索確定時に searchFwd を保存（n/N の方向ずれ防止）
4	🟡 Minor	model.go	非キーMsg（textinput.Blink等）を入力欄へ委譲しカーソル点滅を維持
5	🟡 Minor	render.go	highlightMatch をrune境界ベースに変更（マルチバイト分割防止）
6	🟡 Minor	docs/design.md	表セル内 ghq list | fzf の | をエスケープ
7	🟠 Major(doc)	docs/design.md	前提条件の fzf を任意ツール扱いに修正
8	🟡 Minor	CLAUDE.md	ワークフロー番号 1→2→3→4→5→6 に修正
9	🔵 Trivial	selector.go / model.go	ListRepositories() を tea.Cmd で非同期化（repoListMsg 導入）

追加クリーンアップ

• 誤ってコミットされていた nix ビルド成果物 result（シンボリックリンク）を削除し、.gitignore に追加

テスト

• 後方検索・同一パスガード・マルチバイトハイライト・セレクタ非同期取得のテストを追加
• gofmt クリーン / go vet OK / golangci-lint 0 issues / go test ./... 全パス

🤖 Generated with Claude Code

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ℹ️ Review info

⚙️ Run configuration

Configuration used: Path: .coderabbit.yaml

Review profile: CHILL

Plan: Pro

Run ID: e1f1beef-f4ea-43cc-8a92-9ea539126e3a

📥 Commits

Reviewing files that changed from the base of the PR and between d6f894c and 7616a4f.

📒 Files selected for processing (10)

• .gitignore
• CLAUDE.md
• docs/design.md
• internal/ui/keybinding.go
• internal/ui/model.go
• internal/ui/model_test.go
• internal/ui/render.go
• internal/ui/render_test.go
• internal/ui/selector.go
• result

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch fix/coderabbit-review-pr14

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