feat(bin): support bun fallback runtime for codex CLI (#282)

This PR adds a shell wrapper in `codex-cli/bin/codex` to detect node or
bun as the runtime.

It updates:
- `package.json` bin entry
- published files list to include bin/
- README install instructions to include `bun install -g @openai/codex`

codex-cli/bin/codex

@@ -0,0 +1,20 @@
+#!/usr/bin/env sh
+# resolve script path in case of symlink
+SOURCE="$0"
+while [ -h "$SOURCE" ]; do
+  DIR=$(dirname "$SOURCE")
+  SOURCE=$(readlink "$SOURCE")
+  case "$SOURCE" in
+    /*) ;;  # absolute path
+    *) SOURCE="$DIR/$SOURCE" ;;  # relative path
+  esac
+done
+DIR=$(cd "$(dirname "$SOURCE")" && pwd)
+if command -v node >/dev/null 2>&1; then
+  exec node "$DIR/../dist/cli.js" "$@"
+elif command -v bun >/dev/null 2>&1; then
+  exec bun "$DIR/../dist/cli.js" "$@"
+else
+  echo "Error: node or bun is required to run codex" >&2
+  exit 1
+fi