Feature Request: User-defined tags on sessions (searchable and filterable)

### Describe the feature or problem you'd like to solve

As the number of active sessions grows, it becomes difficult to organize and find specific sessions. Currently there's no way to categorize sessions beyond their name. Users working across multiple features, repos, or workstreams need a lightweight way to group and locate sessions.

### Proposed solution

Allow users to add custom tags to sessions. Tags should be:

- **User-defined**: Free-form text labels (e.g., `auth`, `frontend`, `bugfix`, `v2.1`, `urgent`)
- **Displayed on session items**: Shown as compact colored badges/chips on each session in the sidebar list
- **Searchable**: A search/filter input in the session list that matches against tag names
- **Filterable**: Ability to filter the session list to show only sessions with a specific tag (click-to-filter or dropdown)
- **Multi-tag support**: A session can have multiple tags

**Workflow:**
1. User right-clicks or uses a menu on a session → "Add tag"
2. Autocomplete suggests previously-used tags, or user types a new one
3. Tags appear as small chips on the session item in the sidebar
4. User clicks a tag (or uses a filter bar) to narrow the session list to only matching sessions
5. A search box matches against session names AND tags

**Optional enhancements:**
- Tag color assignment (auto or manual)
- Pinned/favorite tags for quick filtering
- Tag management view (rename, merge, delete unused tags)
- Keyboard shortcut to add/search tags

### Example prompts or workflows

1. User is working on a large project with sessions for auth, payments, and UI
2. They tag sessions: `auth`, `payments`, `ui`
3. Later, they click the `payments` tag filter → sidebar shows only the 3 payment-related sessions
4. They search "bugfix" → shows all sessions tagged `bugfix` regardless of session name

### Additional context

This is complementary to #3969 (plan status indicators). Together, tags + status indicators would give users powerful at-a-glance organization and progress visibility across many sessions.

Issue では、実装ファイル、テスト、エントリポイントが特定されていません。まずセッションサイドバー、セッション検索、セッション永続化のパスを特定し、次にタグの追加、保存、表示、検索、フィルタリングの方法を定義します。完了の条件は、複数タグを持つセッションが説明されたワークフローをサポートすることです。

索引モデルが issue の本文から書いたものです。