Add shared fallback environments

Author: linhay

.skills/swift-git/SKILL.md

@@ -51,6 +51,15 @@ _ = try await git.clone([.progress], repository: url, directory: "/tmp/repo") {
 }
 ```
 
+Shared fallback configuration (manual)
+```swift
+import SwiftGit
+import SwiftGitArm64
+
+Git.configureShared(environments: [.embed(.arm64), .system])
+let git = try Git.shared
+```
+
 Repository constraints (MUST follow)
 - Do NOT create commits or push to remote on behalf of a user unless explicitly authorized.
 - Do NOT modify the embedded git bundle at Sources/SwiftGit/Resource/git-instance.bundle.

README.md

@@ -83,6 +83,7 @@ Important modules (quick code map)
 
 - `Git` (`Sources/SwiftGit/Custom/Git.swift`)
   - High-level entrypoint. Use `Git(environment:)` or `try Git.shared` (throws if environment fails).
+  - Configure shared fallback environments with `Git.configureShared(environments:)`.
   - Methods: `run`, `runPublisher`, `data` with variants for arrays of args and string commands.
 
 - `Repository` (`Sources/SwiftGit/Custom/Repository.swift` + `Repository+*.swift`)
@@ -108,6 +109,16 @@ import SwiftGitArm64
 
 let git = Git(environments: [.embed(.arm64), .system])
 ```
+
+Shared configuration (arm64):
+
+```swift
+import SwiftGit
+import SwiftGitArm64
+
+Git.configureShared(environments: [.embed(.arm64), .system])
+let git = try Git.shared
+```
 - Agents and tests should not modify files under the bundles. Use `GitEnvironment.Style.custom(URL)` to point to an alternate git distribution.
 - To generate updated bundles, run `tools/update-git-bundle.sh` with the desired `--archs` and copy the result into the corresponding `Sources/SwiftGitResources*/Resource/` directory.
 - If a bundle looks unusually large, run `tools/fix-git-bundle-links.sh` to relink duplicate `git-*` binaries and shrink the footprint.

Sources/SwiftGit/Custom/Git.swift

@@ -11,10 +11,19 @@ import Foundation
 public class Git {
 
     public static var _shared: Git?
+    private static var _sharedEnvironments: [GitEnvironment]?
+
+    public static func configureShared(environments: [GitEnvironment]) {
+        precondition(!environments.isEmpty, "Git.shared requires at least one GitEnvironment")
+        _sharedEnvironments = environments
+        _shared = nil
+    }
+
     public static var shared: Git {
         get throws {
             if let git = _shared { return git }
-            let git = try Git(environment: .shared)
+            let environments = _sharedEnvironments ?? [try GitEnvironment.shared]
+            let git = Git(environments: environments)
             _shared = git
             return git
         }