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/event_dispatch.rs

@@ -1848,6 +1848,10 @@ impl App {
                 self.chat_widget.set_status_line_branch(cwd, branch);
                 self.refresh_status_line();
             }
+            AppEvent::StatusLineGitSummaryUpdated { cwd, summary } => {
+                self.chat_widget.set_status_line_git_summary(cwd, summary);
+                self.refresh_status_line();
+            }
             AppEvent::StatusLineSetupCancelled => {
                 self.chat_widget.cancel_status_line_setup();
             }

codex-rs/tui/src/app/test_support.rs

@@ -19,6 +19,7 @@ pub(super) async fn make_test_app() -> App {
         session_telemetry,
         app_event_tx,
         chat_widget,
+        workspace_command_runner: None,
         config,
         state_db: None,
         active_profile: None,

codex-rs/tui/src/app/tests.rs

@@ -424,6 +424,7 @@ async fn enqueue_primary_thread_session_replays_turns_before_initial_prompt_subm
         config,
         frame_requester: crate::tui::FrameRequester::test_dummy(),
         app_event_tx: app.app_event_tx.clone(),
+        workspace_command_runner: None,
         initial_user_message: create_initial_user_message(
             Some(initial_prompt.clone()),
             Vec::new(),
@@ -3770,6 +3771,7 @@ async fn make_test_app() -> App {
         session_telemetry,
         app_event_tx,
         chat_widget,
+        workspace_command_runner: None,
         config,
         state_db: None,
         active_profile: None,
@@ -3832,6 +3834,7 @@ async fn make_test_app_with_channels() -> (
             session_telemetry,
             app_event_tx,
             chat_widget,
+            workspace_command_runner: None,
             config,
             state_db: None,
             active_profile: None,
@@ -4733,6 +4736,7 @@ async fn replace_chat_widget_reseeds_collab_agent_metadata_for_replay() {
         config: app.config.clone(),
         frame_requester: crate::tui::FrameRequester::test_dummy(),
         app_event_tx: app.app_event_tx.clone(),
+        workspace_command_runner: None,
         initial_user_message: None,
         enhanced_keys_supported: app.enhanced_keys_supported,
         has_chatgpt_account: app.chat_widget.has_chatgpt_account(),