Diff view: see what the agent changed

[karngyan]

Why

After the terminal itself, the thing you most want from a phone is "what did it change". Today that is scrolling back through tool output, or typing git diff into the session and reading a pager on a 6 inch screen. A real diff view, split or unified, per file, with syntax highlighting, is the review half of supervising an agent. It is also where Pierre's @pierre/diffs earns a place in flue, if #129 says yes.

Rough scope

• A wire verb that streams a patch the way read streams a file:

  diff{id, path?, staged?, reqId}   ------->  git diff in the session cwd
       <-------  file{ref, ...}  0x02 chunks  eof{ref}
  

  Same chunking, same caps, same cancel. The daemon runs git as the user, nothing else.

• UI: a changed-files list for the session's working directory (from the same git status --porcelain=v2 the tree uses in Fast directory listing over the wire for big repos #128), tap a file, see its diff. Unified on phones, split on desktop. Own route next to /file and /files so it opens in a tab.

• Untracked files: no diff exists, so synthesize one against empty, or just open the file in the viewer. Binary files: say so.

• Later, once reading works: stage and commit from the phone, and sending a line comment back into the agent session as a prompt. Those are separate tickets; this one reads.

Hard parts, named up front

• Huge diffs. An agent that rewrote 400 files produces a patch that must be virtualized, not rendered. This is the exact case the Pierre renderer exists for, and the reason Spike: Pierre's @pierre/diffs and @pierre/trees in flue #129 matters.
• Renames and mode changes, submodules, and worktrees where the session cwd is not the repository root. git handles them; the UI has to not fall over when it does.
• Same CSP and one-shiki rules as the viewer.

Notes

• Sibling of Code browser: file tree and viewer for a session's working directory #127; shares the tree and the viewer but is a separate feature with its own verb.
• Pairs with GitHub PR and CI status on sessions #117 (GitHub PR and CI status on sessions): the local diff is what you review before the PR exists.