Git integration: track commit & branch in eval runs

[mattpocock]

Problem

Currently evalite doesn't track which git commit or branch an eval was run against. This makes it hard to:

• Correlate eval results with specific code versions
• Track performance across commits/branches
• Understand which code changes affected eval scores

Proposal

Store in eval run metadata:

• Commit SHA
• Branch name
• (Optional) dirty state flag

Implementation considerations

• Read git info at run start (similar to how we get cwd)
• Store in runs table (add commit & branch columns)
• Display in UI (run history, comparison views)
• Handle non-git repos gracefully
• Handle dirty working trees

Scope

Likely post-v1 - fairly tasty chunk of work that could expose funny bugs.

Questions

• Should we block runs if working tree is dirty?
• Should we auto-compare across branches?
• Git info in filesystem mode vs server mode?