feat(tui): add PR summary statusline items (#20892)

## Why?

The Codex App already exposes branch and PR context in its
branch-details UI. This brings the same context into the CLI footer as
opt-in statusline items, so users can choose the extra signal without
making the default footer busier.

## What?

Add optional `pull-request-number` and `branch-changes` items to the
configurable TUI status line.

- `pull-request-number` shows the open PR for the current checkout and
renders as a clickable terminal hyperlink when OSC 8 links are
supported.
- `branch-changes` shows committed additions/deletions against the
repository default branch, or `No changes` when the branch has no
committed diff.

<img width="1257" height="261" alt="CleanShot 2026-05-03 at 20 44 15"
src="https://github.com/user-attachments/assets/10b4380b-c3e9-4729-9ee1-3f742068fa47"
/>

## Architecture

This follows the same client/app-server split as the Codex App: the TUI
owns presentation, caching, and optional rendering, while
workspace-sensitive `git` and `gh` discovery runs through app-server.

The new TUI-local `workspace_command` layer sends bounded,
non-interactive `command/exec` requests to the active app-server. That
makes the implementation remote-friendly: the TUI does not decide
whether commands run in an embedded local workspace or a remote
workspace, and it does not bypass app-server sandbox or permission
policy.

The branch summary logic stays internal to `codex-tui` because this PR
only needs TUI statusline behavior. The command boundary is still
isolated behind `WorkspaceCommandExecutor`, so the lookup code can be
lifted or reused later without changing statusline rendering.

## How?

- Add a TUI `WorkspaceCommandExecutor` abstraction backed by app-server
`command/exec`.
- Add branch summary probes for:
  - current branch name,
  - open PR metadata,
  - committed branch diff stats against the default branch.
- Prefer remote-tracking default branch refs for diff stats, avoiding
stale or absent local `main` branches.
- Resolve PRs with `gh pr view` first, then fall back to
commit-associated PR lookup across parent/fork repos.
- Add `/statusline` picker entries, preview values, rendering, and OSC 8
clickable PR links.
- Keep all probes best-effort so missing `git`, missing `gh`, auth
failures, or non-git directories hide optional items instead of
surfacing footer errors.

## Validation

- `cargo test -p codex-tui branch_summary -- --nocapture`
- Snapshot coverage for the `/statusline` preview/setup rendering paths
- Hyperlink rendering coverage for clickable PR statusline cells

codex-rs/tui/src/app.rs

@@ -76,6 +76,8 @@ use crate::tui;
 use crate::tui::TuiEvent;
 use crate::update_action::UpdateAction;
 use crate::version::CODEX_CLI_VERSION;
+use crate::workspace_command::AppServerWorkspaceCommandRunner;
+use crate::workspace_command::WorkspaceCommandRunner;
 use codex_ansi_escape::ansi_escape_line;
 use codex_app_server_client::AppServerRequestHandle;
 use codex_app_server_client::TypedRequestError;
@@ -432,6 +434,7 @@ pub(crate) struct App {
     pub(crate) session_telemetry: SessionTelemetry,
     pub(crate) app_event_tx: AppEventSender,
     pub(crate) chat_widget: ChatWidget,
+    workspace_command_runner: Option<WorkspaceCommandRunner>,
     /// Config is stored here so we can recreate ChatWidgets as needed.
     pub(crate) config: Config,
     pub(crate) state_db: Option<StateDbHandle>,
@@ -574,6 +577,7 @@ impl App {
             config: cfg,
             frame_requester: tui.frame_requester(),
             app_event_tx: self.app_event_tx.clone(),
+            workspace_command_runner: self.workspace_command_runner.clone(),
             initial_user_message,
             enhanced_keys_supported: self.enhanced_keys_supported,
             has_chatgpt_account: self.chat_widget.has_chatgpt_account(),
@@ -712,6 +716,9 @@ impl App {
 
         let status_line_invalid_items_warned = Arc::new(AtomicBool::new(false));
         let terminal_title_invalid_items_warned = Arc::new(AtomicBool::new(false));
+        let workspace_command_runner: WorkspaceCommandRunner = Arc::new(
+            AppServerWorkspaceCommandRunner::new(app_server.request_handle()),
+        );
         let runtime_model_provider_base_url =
             resolve_runtime_model_provider_base_url(&config.model_provider).await;
 
@@ -734,6 +741,7 @@ impl App {
                     config: config.clone(),
                     frame_requester: tui.frame_requester(),
                     app_event_tx: app_event_tx.clone(),
+                    workspace_command_runner: Some(workspace_command_runner.clone()),
                     initial_user_message: crate::chatwidget::create_initial_user_message(
                         initial_prompt.clone(),
                         initial_images.clone(),
@@ -769,6 +777,7 @@ impl App {
                     config: config.clone(),
                     frame_requester: tui.frame_requester(),
                     app_event_tx: app_event_tx.clone(),
+                    workspace_command_runner: Some(workspace_command_runner.clone()),
                     initial_user_message: crate::chatwidget::create_initial_user_message(
                         initial_prompt.clone(),
                         initial_images.clone(),
@@ -809,6 +818,7 @@ impl App {
                     config: config.clone(),
                     frame_requester: tui.frame_requester(),
                     app_event_tx: app_event_tx.clone(),
+                    workspace_command_runner: Some(workspace_command_runner.clone()),
                     initial_user_message: crate::chatwidget::create_initial_user_message(
                         initial_prompt.clone(),
                         initial_images.clone(),
@@ -856,6 +866,7 @@ See the Codex keymap documentation for supported actions and examples."
             session_telemetry: session_telemetry.clone(),
             app_event_tx,
             chat_widget,
+            workspace_command_runner: Some(workspace_command_runner),
             config,
             state_db,
             active_profile,