amazonq: Automatically generate commit messages

## Problem

I need the ability to modify code in VS Code source control and automatically generate a summary of the changes before committing.

## Expected behavior

In the commit message entry window, there is a Summarize button, which when pressed, generates and automatically fills in a summary of code changes based on the changed code.

The issue names the VS Code source-control commit message entry window but no files or tests. Start by locating the extension's source-control integration and existing Amazon Q summarization entry points. Done means a Summarize button fills the commit message with a summary based on the changed code.

Written by the indexing model from the issue text.