Add Rust install instructions and pre-commit cargo build hook

.pre-commit-config.yaml

@@ -6,4 +6,10 @@ repos:
         entry: python3 agentydragon/tools/check_task_frontmatter.py
         language: python
         additional_dependencies: [PyYAML]
-        files: ^agentydragon/tasks/[0-9]{2}-.*\.md$
+        files: ^agentydragon/tasks/[0-9]{2}-.*\.md$
+      - id: cargo-build
+        name: Check Rust workspace builds
+        entry: bash -lc 'cd codex-rs && cargo build --workspace --locked'
+        language: system
+        pass_filenames: false
+        require_serial: true