feat(cli): add --diff flag to show staged changes in editor

[takai]

Summary

• Add -d / --diff CLI flag that passes --verbose to git commit when in edit mode, showing the staged diff in the editor so the user can verify the AI-generated message against actual code changes
• --diff implies --edit — using it alone automatically opens the editor
• Maps to git's native --verbose flag for seamless integration

Test plan

• go build ./... passes
• go test ./... passes (3 new test cases for --diff, -d, -de cluster)
• Manual: git ai-commit --diff opens editor with diff visible below commit message
• Manual: git ai-commit -de opens editor with diff (cluster shorthand)