Add inline diff viewer for session file changes

[bencered]

The session detail view shows which files were changed (+N/-N stats in the right sidebar) but there's no way to see the actual diff content. Users have to go to GitHub to see what the agent changed.

Should add a diff viewer tab to the session detail page. The Diffs.com library is great for this. Only issue is that it doesn't support adding review comments, which would be nice to have for steering the agent.

[ColeMurray]

onboard with adding this!

if you or someone else wants to follow-up with a plan for implementing this, let's align and then can proceed with implementation

[bencered]

I'd like to do a bit of a UI refresh (migrate to shadcn, follow next.js best practices, clarify some design elements), and this would be a great sub-issue. The final result would look something similar to this:

A good stretch goal would be the ability to add comments to the diff, but that'd work better as a separate issue IMO. And diffs.com doesn't support it natively afaik.

For the diff capturing, we could run git diff origin/<baseBranch> after each tool call (debounced) to capture the diff in real time. Then store that in D1 (or R2?) under the session table.

[bencered]

This is a quick (functional) mockup of what the diff would look like with comments. Supports single- and multi-line comments.

[Astro-Han]

Built diffpane around this workflow, in case it is useful as a reference.

The main design choice that ended up mattering was using a session baseline: snapshot HEAD when review starts, then diff against that baseline continuously. That keeps the review focused on "what changed in this agent session" instead of mixing in unrelated repo state.

It also auto-follows the latest changed file, which made multi-file agent runs much easier to audit in practice. Not suggesting this as the exact implementation for Background Agents, but the session-baseline model may be a good fit for the diff tab you are discussing.