Cache Codex Lab app builds on artifact volume

.github/workflows/README.md

@@ -41,6 +41,26 @@ The current local runner is `chris-mac-codex-release-1`. It must have Rust,
 Python 3, Xcode command line tools, and macOS `ditto` available. The generated
 Codex Lab app artifact is currently unsigned.
 
+### Developer Artifacts Volume
+
+High-churn runner data should live under `/Volumes/Developer-Artifacts` when
+that volume is mounted. Keep the layout stable and purpose-based so future
+builds, local automation, and cleanup scripts can share the volume without
+guessing what owns each path:
+
+- `/Volumes/Developer-Artifacts/github-actions/runners/` for self-hosted runner
+  installations.
+- `/Volumes/Developer-Artifacts/github-actions/cache/` for reusable caches that
+  should survive checkout cleanup.
+- `/Volumes/Developer-Artifacts/github-actions/tmp/` for disposable workflow
+  scratch data that can be removed without losing build acceleration.
+
+Workflow-specific caches should add owner/repo and workflow leaves under
+`github-actions/cache/`. For example, `codex-lab-app.yml` uses
+`/Volumes/Developer-Artifacts/github-actions/cache/<owner>/<repo>/codex-lab-app/`
+as its Cargo target cache root on self-hosted runners, falling back to Cargo's
+default target directory when the artifact volume is unavailable.
+
 ## Codex Lab Distribution Contract
 
 `codex-lab-app.yml` uploads these files in one artifact:

.github/workflows/codex-lab-app.yml

@@ -36,6 +36,22 @@ jobs:
 
       - uses: dtolnay/rust-toolchain@e081816240890017053eacbb1bdf337761dc5582 # 1.95.0
 
+      - name: Use persistent artifact-volume build cache
+        shell: bash
+        run: |
+          set -euo pipefail
+          if [ "${RUNNER_ENVIRONMENT:-}" = "self-hosted" ] && [ -d /Volumes/Developer-Artifacts ]; then
+            cache_root="/Volumes/Developer-Artifacts/github-actions/cache/${GITHUB_REPOSITORY}/codex-lab-app"
+            target_dir="${cache_root}/cargo-target-aarch64-apple-darwin-release"
+            mkdir -p "$target_dir"
+            echo "CARGO_TARGET_DIR=$target_dir" >> "$GITHUB_ENV"
+            echo "CODEX_LAB_BIN=$target_dir/release/codex-lab" >> "$GITHUB_ENV"
+            echo "Using persistent artifact-volume target cache: $target_dir"
+          else
+            echo "CODEX_LAB_BIN=codex-rs/target/release/codex-lab" >> "$GITHUB_ENV"
+            echo "Using default Cargo target directory"
+          fi
+
       - name: Build Codex Lab CLI
         working-directory: codex-rs
         shell: bash
@@ -52,7 +68,7 @@ jobs:
           lab_version="$(PYTHONPATH=scripts python3 -c 'from codex_package.version import read_workspace_version; print(read_workspace_version())')"
           mkdir -p "$output_root"
           python3 scripts/build_codex_lab_app.py \
-            --codex-bin codex-rs/target/release/codex-lab \
+            --codex-bin "$CODEX_LAB_BIN" \
             --app-dir "$app_dir" \
             --shim-dir "$shim_dir" \
             --short-version "$lab_version" \