feat(linux-sandbox): add bwrap support (#9938)

## Summary
This PR introduces a gated Bubblewrap (bwrap) Linux sandbox path. The
curent Linux sandbox path relies on in-process restrictions (including
Landlock). Bubblewrap gives us a more uniform filesystem isolation
model, especially explicit writable roots with the option to make some
directories read-only and granular network controls.

This is behind a feature flag so we can validate behavior safely before
making it the default.

- Added temporary rollout flag:
  - `features.use_linux_sandbox_bwrap`
- Preserved existing default path when the flag is off.
- In Bubblewrap mode:
- Added internal retry without /proc when /proc mount is not permitted
by the host/container.

.github/workflows/rust-release.yml

@@ -65,6 +65,8 @@ jobs:
     defaults:
       run:
         working-directory: codex-rs
+    env:
+      CODEX_BWRAP_ENABLE_FFI: ${{ contains(matrix.target, 'unknown-linux') && '1' || '0' }}
 
     strategy:
       fail-fast: false
@@ -89,6 +91,13 @@ jobs:
 
     steps:
       - uses: actions/checkout@v6
+      - name: Install Linux bwrap build dependencies
+        if: ${{ runner.os == 'Linux' }}
+        shell: bash
+        run: |
+          set -euo pipefail
+          sudo apt-get update -y
+          sudo DEBIAN_FRONTEND=noninteractive apt-get install -y --no-install-recommends pkg-config libcap-dev
       - name: Install UBSan runtime (musl)
         if: ${{ matrix.target == 'x86_64-unknown-linux-musl' || matrix.target == 'aarch64-unknown-linux-musl' }}
         shell: bash