(autofix) Add commit messages to PR creation

[roaga]

As an example, take this PR:

Each file change is just titled "File Change" instead of a more useful description of the change. Luckily, we already ask Autofix to create commit messages that are stored in FileChange objects.

The problem was introduced in this PR to support user-editing of the outputted diff in the UI. Since that diff was stored as a FilePatch, converted from a FileChange, it was easier at the time to directly create the user-edited PR from the FilePatch objects directly. This lost the commit messages though because the outputted diff was not mapped to the original FileChange objects and their associated commits.

See src/seer/automation/autofix/tasks.py > update_code_change and run_autofix_create_pr for a good starting point for when these flows are triggered. Also check out src/seer/automation/codebase/repo_client.py to see how we interface with GitHub and create PRs, specifically in create_branch_from_changes. Finally check out src/seer/automation/models.py to understand the FilePatch and FileChange objects.