add collapsible sections to sidebar

[samguyette-xyz]

Adds chevron buttons to sidebar section headers (CHANGED, REVIEWED, REFERENCE, COMMENTS, AGENT) allowing users to collapse sections to save space. This solves the issue where PRs with many changed files make it difficult to scroll to the agent section.

Also, maximize agent response size:

[wesbillman]

@samguyette-xyz I think the code has changed dramatically so this could be a bit tricky to merge in. Happy to try to get through a rebase here, or we can add an issue and reimplement from the current staged. I do think this is a good feature though. Just worried we've made it tricky to merge 😬

Whatcha think?

[samguyette-xyz]

@wesbillman I’ll pull down the latest version and take a look at where things stand. Staged has been moving quickly, so that tracks 👍