feat(tui): route /diff through workspace commands (#21001)

Stacked on #20892.

## Why

#20892 adds the TUI workspace command abstraction so branch status
metadata can run through app-server instead of assuming the CLI process
has the active workspace locally. `/diff` still used direct local
process execution, which means remote app-server sessions could compute
the diff against the wrong machine or fail to see the active workspace
at all.

This PR moves `/diff` onto that same app-server-backed command path so
Git runs wherever the active workspace lives.

## What Changed

- Route `/diff` through the TUI `WorkspaceCommandExecutor` using the
active chat cwd.
- Replace direct `tokio::process::Command` usage in `get_git_diff` with
argv-based workspace command requests.
- Preserve the existing `/diff` behavior: tracked diff output, untracked
file diffs, treating Git diff exit code `1` as success, and showing the
existing non-git-repository message.
- Extend `WorkspaceCommand` with caller-set timeouts and an explicit
uncapped-output opt-out. Metadata probes remain capped by default;
`/diff` opts out because its full output is the user-visible payload.

## How to Test

Manual reviewer path:

1. Start the Codex TUI from a Git worktree with one tracked file change
and one untracked file.
2. Run `/diff`.
3. Confirm the rendered diff includes both the tracked diff and the
untracked file diff.
4. Start the TUI outside a Git worktree, or switch to a non-git cwd,
then run `/diff`.
5. Confirm it shows the existing `/diff` not-inside-a-git-repository
message.

Targeted tests run:

- `cargo test -p codex-tui get_git_diff -- --nocapture`
- `cargo test -p codex-tui branch_summary -- --nocapture`
- `cargo test -p codex-tui`

codex-rs/tui/src/chatwidget/slash_dispatch.rs

@@ -328,16 +328,25 @@ impl ChatWidget {
             SlashCommand::Diff => {
                 self.add_diff_in_progress();
                 let tx = self.app_event_tx.clone();
+                let runner = self.workspace_command_runner.clone();
+                let cwd = self
+                    .current_cwd
+                    .clone()
+                    .unwrap_or_else(|| self.config.cwd.to_path_buf());
                 tokio::spawn(async move {
-                    let text = match get_git_diff().await {
-                        Ok((is_git_repo, diff_text)) => {
-                            if is_git_repo {
-                                diff_text
-                            } else {
-                                "`/diff` — _not inside a git repository_".to_string()
+                    let text = match runner {
+                        Some(runner) => match get_git_diff(runner.as_ref(), &cwd).await {
+                            Ok((is_git_repo, diff_text)) => {
+                                if is_git_repo {
+                                    diff_text
+                                } else {
+                                    "`/diff` — _not inside a git repository_".to_string()
+                                }
                             }
-                        }
-                        Err(e) => format!("Failed to compute diff: {e}"),
+                            Err(e) => format!("Failed to compute diff: {e}"),
+                        },
+                        None => "Failed to compute diff: workspace command runner unavailable"
+                            .to_string(),
                     };
                     tx.send(AppEvent::DiffResult(text));
                 });