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/core/src/config/mod.rs

@@ -140,12 +140,30 @@ pub(crate) const DEFAULT_AGENT_JOB_MAX_RUNTIME_SECONDS: Option<u64> = None;
 
 pub const CONFIG_TOML_FILE: &str = "config.toml";
 const OPENAI_BASE_URL_ENV_VAR: &str = "OPENAI_BASE_URL";
+#[cfg(target_os = "linux")]
+const SYSTEM_BWRAP_PATH: &str = "/usr/bin/bwrap";
 const RESERVED_MODEL_PROVIDER_IDS: [&str; 3] = [
     OPENAI_PROVIDER_ID,
     OLLAMA_OSS_PROVIDER_ID,
     LMSTUDIO_OSS_PROVIDER_ID,
 ];
 
+#[cfg(target_os = "linux")]
+pub fn missing_system_bwrap_warning() -> Option<String> {
+    if Path::new(SYSTEM_BWRAP_PATH).is_file() {
+        None
+    } else {
+        Some(format!(
+            "Codex could not find system bubblewrap at {SYSTEM_BWRAP_PATH}. Please install bubblewrap with your package manager. Codex will use the vendored bubblewrap in the meantime."
+        ))
+    }
+}
+
+#[cfg(not(target_os = "linux"))]
+pub fn missing_system_bwrap_warning() -> Option<String> {
+    None
+}
+
 fn resolve_sqlite_home_env(resolved_cwd: &Path) -> Option<PathBuf> {
     let raw = std::env::var(codex_state::SQLITE_HOME_ENV).ok()?;
     let trimmed = raw.trim();