Feature Request: Built-in text/content search across files

## Feature Request

**Summary:**
GitUI currently does not provide a built-in way to search for text or content across files in a repository. Users who want to find a specific string, pattern, or piece of code must leave the GitUI interface and use an external tool (e.g. `grep`, `rg`, or their editor's search).

**Expected Behavior:**
GitUI should include a built-in file content search (similar to `grep` / `ripgrep`) that allows users to:
- Enter a search query (plain text or regex)
- See matching files and line previews directly within the TUI
- Navigate to a specific match in context

**Why This Matters:**
Code review and diff inspection workflows often require looking up where a symbol or string is used across the repository. Having to switch out of GitUI to a separate terminal search breaks the flow and reduces the tool's usefulness as a self-contained Git UI.

**Possible Approach:**
- Add a dedicated search panel (e.g. reachable via a keybinding like `/` or a dedicated shortcut)
- Internally delegate to `git grep` or an optional `ripgrep`/`grep` backend
- Display results in a scrollable list with file path, line number, and a snippet of the matching line
- Allow jumping into the diff/file view from a result

**Alternatives Considered:**
- Using `git grep` manually in a separate terminal — works but requires context switching
- Using editor search — not available when GitUI is the primary interface

**References:**
- Similar feature in other TUI Git clients (e.g. lazygit's filter/search capabilities)

ファイルやテストは特定されていないため、まず既存のTUIエントリーポイントにあるキーバインド、パネル、diff/ファイルビューを調査します。提案されている `/` ショートカットを中心に検索パネルを定義し、`git grep` またはオプションのripgrep/grepバックエンドを評価します。ユーザーがテキストまたはregexを検索し、行プレビュー付きでファイルパスを閲覧し、コンテキスト内の一致箇所に移動できれば完了です。

索引モデルが issue の本文から書いたものです。