feat: ultra polish package manager (#13573)

See the readme

codex-rs/package-manager/src/config.rs

@@ -0,0 +1,40 @@
+use crate::ManagedPackage;
+use std::path::PathBuf;
+
+/// Immutable configuration for a [`crate::PackageManager`] instance.
+#[derive(Clone, Debug, PartialEq, Eq)]
+pub struct PackageManagerConfig<P> {
+    pub(crate) codex_home: PathBuf,
+    pub(crate) package: P,
+    cache_root: Option<PathBuf>,
+}
+
+impl<P> PackageManagerConfig<P> {
+    /// Creates a config rooted at the provided Codex home directory.
+    pub fn new(codex_home: PathBuf, package: P) -> Self {
+        Self {
+            codex_home,
+            package,
+            cache_root: None,
+        }
+    }
+
+    /// Overrides the package cache root instead of deriving it from `codex_home`.
+    pub fn with_cache_root(mut self, cache_root: PathBuf) -> Self {
+        self.cache_root = Some(cache_root);
+        self
+    }
+}
+
+impl<P: ManagedPackage> PackageManagerConfig<P> {
+    /// Returns the effective cache root for the package.
+    pub fn cache_root(&self) -> PathBuf {
+        self.cache_root.clone().unwrap_or_else(|| {
+            self.codex_home.join(
+                self.package
+                    .default_cache_root_relative()
+                    .replace('/', std::path::MAIN_SEPARATOR_STR),
+            )
+        })
+    }
+}