The Ctrl+D diff drawer in `coder exp agents` only rendered PR-backed
diffs returned by `/api/experimental/chats/{id}/diff`. Local working
tree changes in a chat's workspace returned an empty diff, so the
drawer showed "No diff contents" with no file summary.
Centralise diff loading behind a single `fetchChatDiffContents` helper
that first hits `/diff`, then falls back to the chat git watcher
WebSocket (`/stream/git`) when the remote diff is empty. Aggregate the
agent's `WorkspaceAgentRepoChanges` into a `ChatDiffContents` value so
the drawer can derive the file summary and styled body from the local
unified diff. Missing workspaces, missing agents, and watcher timeouts
are treated as graceful fallbacks that render the empty-diff
placeholder instead of a hard error.
> Mux is opening this PR on Mike's behalf.
Michael Suchacz
7904bed947