Diff mode (two files, inline notation, or git)

Use Monaco's built-in **diff editor** (`monaco.editor.createDiffEditor`) — a big reason to keep Monaco:
- `--diff ` — side-by-side diff
- `--inline-diff` — inline (unified) rendering
- `--git ` — snapshot a file's diff against a git ref

- [ ] Two-file diff via `createDiffEditor`
- [ ] Inline vs side-by-side option
- [ ] git integration (read the blob at ``)
- [ ] Reuse the offline Monaco from #6

Depends on #6 (local Monaco).

Start with #6 for the offline Monaco prerequisite, then read the Monaco createDiffEditor entry point. Implement the two-file, inline or side-by-side, and git-ref modes listed in the issue, and confirm that all checklist items work with the specified CLI options.

由索引模型根據 Issue 內容生成。