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/onboarding/auth.rs

@@ -61,6 +61,24 @@ use crate::tui::FrameRequester;
 /// row boundary, which breaks normal terminal URL detection for long URLs that
 /// wrap across multiple rows.
 pub(crate) fn mark_url_hyperlink(buf: &mut Buffer, area: Rect, url: &str) {
+    mark_hyperlink_cells(buf, area, url, |cell| {
+        cell.fg == Color::Cyan && cell.modifier.contains(Modifier::UNDERLINED)
+    });
+}
+
+/// Marks any underlined buffer cells as an OSC 8 hyperlink.
+pub(crate) fn mark_underlined_hyperlink(buf: &mut Buffer, area: Rect, url: &str) {
+    mark_hyperlink_cells(buf, area, url, |cell| {
+        cell.modifier.contains(Modifier::UNDERLINED)
+    });
+}
+
+fn mark_hyperlink_cells(
+    buf: &mut Buffer,
+    area: Rect,
+    url: &str,
+    should_mark: impl Fn(&ratatui::buffer::Cell) -> bool,
+) {
     // Sanitize: strip any characters that could break out of the OSC 8
     // sequence (ESC or BEL) to prevent terminal escape injection from a
     // malformed or compromised upstream URL.
@@ -75,8 +93,7 @@ pub(crate) fn mark_url_hyperlink(buf: &mut Buffer, area: Rect, url: &str) {
     for y in area.top()..area.bottom() {
         for x in area.left()..area.right() {
             let cell = &mut buf[(x, y)];
-            // Only mark cells that carry the URL's distinctive style.
-            if cell.fg != Color::Cyan || !cell.modifier.contains(Modifier::UNDERLINED) {
+            if !should_mark(cell) {
                 continue;
             }
             let sym = cell.symbol().to_string();