fix(linux-sandbox): prefer system /usr/bin/bwrap when available (#14963)

## Problem
Ubuntu/AppArmor hosts started failing in the default Linux sandbox path
after the switch to vendored/default bubblewrap in `0.115.0`.

The clearest report is in
[#14919](https://github.com/openai/codex/issues/14919), especially [this
investigation
comment](https://github.com/openai/codex/issues/14919#issuecomment-4076504751):
on affected Ubuntu systems, `/usr/bin/bwrap` works, but a copied or
vendored `bwrap` binary fails with errors like `bwrap: setting up uid
map: Permission denied` or `bwrap: loopback: Failed RTM_NEWADDR:
Operation not permitted`.

The root cause is Ubuntu's `/etc/apparmor.d/bwrap-userns-restrict`
profile, which grants `userns` access specifically to `/usr/bin/bwrap`.
Once Codex started using a vendored/internal bubblewrap path, that path
was no longer covered by the distro AppArmor exception, so sandbox
namespace setup could fail even when user namespaces were otherwise
enabled and `uidmap` was installed.

## What this PR changes
- prefer system `/usr/bin/bwrap` whenever it is available
- keep vendored bubblewrap as the fallback when `/usr/bin/bwrap` is
missing
- when `/usr/bin/bwrap` is missing, surface a Codex startup warning
through the app-server/TUI warning path instead of printing directly
from the sandbox helper with `eprintln!`
- use the same launcher decision for both the main sandbox execution
path and the `/proc` preflight path
- document the updated Linux bubblewrap behavior in the Linux sandbox
and core READMEs

## Why this fix
This still fixes the Ubuntu/AppArmor regression from
[#14919](https://github.com/openai/codex/issues/14919), but it keeps the
runtime rule simple and platform-agnostic: if the standard system
bubblewrap is installed, use it; otherwise fall back to the vendored
helper.

The warning now follows that same simple rule. If Codex cannot find
`/usr/bin/bwrap`, it tells the user that it is falling back to the
vendored helper, and it does so through the existing startup warning
plumbing that reaches the TUI and app-server instead of low-level
sandbox stderr.

## Testing
- `cargo test -p codex-linux-sandbox`
- `cargo test -p codex-app-server --lib`
- `cargo test -p codex-tui-app-server
tests::embedded_app_server_start_failure_is_returned`
- `cargo clippy -p codex-linux-sandbox --all-targets`
- `cargo clippy -p codex-app-server --all-targets`
- `cargo clippy -p codex-tui-app-server --all-targets`

codex-rs/tui/src/app.rs

@@ -275,6 +275,16 @@ fn emit_project_config_warnings(app_event_tx: &AppEventSender, config: &Config)
     )));
 }
 
+fn emit_missing_system_bwrap_warning(app_event_tx: &AppEventSender) {
+    let Some(message) = codex_core::config::missing_system_bwrap_warning() else {
+        return;
+    };
+
+    app_event_tx.send(AppEvent::InsertHistoryCell(Box::new(
+        history_cell::new_warning_event(message),
+    )));
+}
+
 #[derive(Debug, Clone, PartialEq, Eq)]
 struct SessionSummary {
     usage_line: String,
@@ -1963,6 +1973,7 @@ impl App {
         let (app_event_tx, mut app_event_rx) = unbounded_channel();
         let app_event_tx = AppEventSender::new(app_event_tx);
         emit_project_config_warnings(&app_event_tx, &config);
+        emit_missing_system_bwrap_warning(&app_event_tx);
         tui.set_notification_method(config.tui_notification_method);
 
         let harness_overrides =