feat: Add Git class for git operations

- Create src/lib/git.ts with Git class
- Implement isInited() method to check git initialization
- Implement lsFiles() method to get git-tracked files
- Refactor FileEditor to use Git class instead of direct simple-git usage

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <[email protected]>

Author: koki-develop

src/lib/git.ts

@@ -0,0 +1,20 @@
+import simpleGit from "simple-git";
+
+export class Git {
+  private git = simpleGit();
+
+  async isInited(): Promise<boolean> {
+    return await this.git
+      .status()
+      .then(() => true)
+      .catch(() => false);
+  }
+
+  async lsFiles(): Promise<string[]> {
+    const gitFiles = await this.git.raw(["ls-files"]);
+    return gitFiles
+      .trim()
+      .split("\n")
+      .filter((file) => file.trim() !== "");
+  }
+}