cstorage: Strip sha256: prefix from image IDs

Image IDs from `podman build --iidfile` include the `sha256:` prefix,
but containers-storage directories use just the hex digest. Strip the
prefix when opening images to support both formats.

Fixes CI failure where `cfsctl oci pull containers-storage:sha256:...`
failed with "image not found".

See containers/skopeo#2750

Assisted-by: OpenCode (Opus 4.5)

Author: cgwalters

crates/cstorage/src/image.rs

@@ -46,10 +46,19 @@ pub struct Image {
 impl Image {
     /// Open an image by ID using fd-relative operations.
     ///
+    /// The ID can be provided with or without a `sha256:` prefix - the prefix
+    /// will be stripped if present, since containers-storage directories use
+    /// just the hex digest.
+    ///
     /// # Errors
     ///
     /// Returns an error if the image directory doesn't exist or cannot be opened.
     pub fn open(storage: &Storage, id: &str) -> Result<Self> {
+        // Strip the sha256: prefix if present - containers-storage directories
+        // use just the hex digest, but image IDs from podman (e.g. via --iidfile)
+        // include the prefix. See https://github.com/containers/skopeo/issues/2750
+        let id = id.strip_prefix("sha256:").unwrap_or(id);
+
         // Open overlay-images directory from storage root
         let images_dir = storage.root_dir().open_dir("overlay-images")?;