Fix #69: Auto-accept GitHub SSH host key on fresh installations

[HalBowman]

Summary

• Added ensureGitHubHostKey function that automatically whitelists GitHub's SSH host key before git clone operations
• The function runs ssh-keyscan github.com to fetch and add the host key to ~/.ssh/known_hosts
• Only adds the key if not already present to avoid unnecessary I/O

Problem

When flock is set up on a new machine for the first time, the first git clone when adding a new repository fails because the SSH host keys are not whitelisted. This requires manual intervention to accept the host key.

Solution

Before performing any SSH-based git clone (in cloneOrGetRepo), the code now ensures GitHub's SSH host key is in the known_hosts file. This prevents "Host key verification failed" errors on fresh installations.

Testing

• Verified the code compiles successfully
• The function is idempotent - it only adds the key if not already present

Fixes #69