feat: support custom SSH user in git auth configuration

- Allow sshUser setting to override default 'git' user in insteadOf config
- When sshUser is empty, default to 'git@github.com'
- When sshUser is provided (e.g., 'customuser'), use 'customuser@github.com'
- Updated tests to verify custom SSH user behavior in auth configuration

Co-Authored-By: Claude:Qwen3-Coder-Next-4bit

src/git-auth-helper.ts

@@ -65,7 +65,11 @@ class GitAuthHelper {
 
     // Instead of SSH URL
     this.insteadOfKey = `url.${serverUrl.origin}/.insteadOf` // "origin" is SCHEME://HOSTNAME[:PORT]
-    this.insteadOfValues.push(`git@${serverUrl.hostname}:`)
+    const sshUser =
+      this.settings.sshUser && this.settings.sshUser.length > 0
+        ? this.settings.sshUser
+        : 'git'
+    this.insteadOfValues.push(`${sshUser}@${serverUrl.hostname}:`)
     if (this.settings.workflowOrganizationId) {
       this.insteadOfValues.push(
         `org-${this.settings.workflowOrganizationId}@github.com:`