feat: prioritize pruning of old SHA when pulling updated tags (#1102)

* feat: prioritize pruning of old SHA when pulling updated tags

When pulling a new version of a tagged image (e.g., ghcr.io/cirruslabs/macos-runner:sonoma),
set the access date of the previous SHA to epoch time (1970-01-01). This ensures that the
old SHA will be prioritized for pruning, even if it was accessed more recently than other
cached images.

This helps manage disk space more efficiently by automatically cleaning up superseded
versions of frequently-updated tagged images.

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

* format

* Review comments

---------

Co-authored-by: Claude <noreply@anthropic.com>

CONTRIBUTING.md

@@ -29,6 +29,7 @@ Table of Contents
 
 1. Code should follow camel case
 2. Code should follow [SwiftFormat](https://github.com/nicklockwood/SwiftFormat#swift-package-manager-plugin) guidelines. You can auto-format the code by running the following command:
+
 ```bash
 swift package plugin --allow-writing-to-package-directory swiftformat --cache ignore .
 ```

Sources/tart/VMStorageOCI.swift

@@ -27,12 +27,12 @@ class VMStorageOCI: PrunableStorage {
     return digest
   }
 
-  func open(_ name: RemoteName) throws -> VMDirectory {
+  func open(_ name: RemoteName, _ accessDate: Date = Date()) throws -> VMDirectory {
     let vmDir = VMDirectory(baseURL: vmURL(name))
 
     try vmDir.validate(userFriendlyName: name.description)
 
-    try vmDir.baseURL.updateAccessDate()
+    try vmDir.baseURL.updateAccessDate(accessDate)
 
     return vmDir
   }
@@ -180,6 +180,10 @@ class VMStorageOCI: PrunableStorage {
       let transaction = SentrySDK.startTransaction(name: name.description, operation: "pull", bindToScope: true)
       let tmpVMDir = try VMDirectory.temporaryDeterministic(key: name.description)
 
+      // Open an existing VM directory corresponding to this name, if any,
+      // marking it as outdated to speed up the garbage collection process
+      _ = try? open(name, Date(timeIntervalSince1970: 0))
+
       // Lock the temporary VM directory to prevent it's garbage collection
       let tmpVMDirLock = try FileLock(lockURL: tmpVMDir.baseURL)
       try tmpVMDirLock.lock()