Add SSH key fallback for git push/pull authentication

[Copilot]

Git push/pull fails with "Authentication failed. Check your SSH keys are added to ssh-agent" even when keys are properly configured. The sandboxed macOS environment doesn't reliably access ssh-agent.

Changes

LibGit2Service.swift

• Added createSSHCredentials() with cascading authentication:
  1. Try ssh-agent (existing behavior, supports encrypted keys)
  2. Fallback to direct key loading from ~/.ssh/ (id_ed25519, id_rsa, id_ecdsa, id_dsa)
• Updated both push() and fetch() credential callbacks
• Added debug logging for authentication attempts

private static func createSSHCredentials(
    out: UnsafeMutablePointer<OpaquePointer?>?,
    username: UnsafePointer<CChar>?
) -> Int32 {
    // Try ssh-agent first
    let agentResult = git_credential_ssh_key_from_agent(out, username)
    if agentResult == 0 { return agentResult }
    
    // Fallback: try direct key files from ~/.ssh
    for keyFile in ["id_ed25519", "id_rsa", "id_ecdsa", "id_dsa"] {
        // Check existence, attempt git_credential_ssh_key_new
    }
}

git-integration.md

• Documented authentication cascade and supported key types
• Added troubleshooting for passphrase-protected vs unencrypted keys

Limitations

Direct key loading only works with unencrypted SSH keys. Passphrase-protected keys require ssh-agent (libgit2 constraint with empty passphrase parameter).

Original prompt

---

This section details on the original issue you should resolve

<issue_title>GitHub push/pull failing; error says "Check your SSH keys are added to ssh-agent."</issue_title>
<issue_description>Error:
Push failed: Authentication failed. Check your SSH keys are added to ssh-agent.

This happens even though the SSH key is indeed added to ssh-agent already.</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes GitHub push/pull failing; error says "Check your SSH keys are added to ssh-agent." #49

---

💬 We'd love your input! Share your thoughts on Copilot coding agent in our 2 minute survey.