Agents window: Want visual indicator that commit is still in progress

Testing #332192

- Working on a session in the Agents window based on a PR
- Send a prompt that would result in some code changes
- Use the action in the titlebar to commit the changes
- ❓ I saw no output (in the agent session, or in the titlebar button area) indicating the commit was working or in progress, so as it worked for a few seconds, I wasn't sure if anything was happening
- After a few seconds, the button did change to `Sync Changes`, so the commit did seem to work, but I think some sort of loading and/or completion indicator would've been nice