Function register_node 

pub fn register_node(
    aida_repo: &Path,
    user_id: u32,
    hostname: &str,
) -> Result<u32>

Register a new node in the aida registry via git CAS push loop.

This is the core distributed identity mechanism:

1. Pull latest from remote
2. Read node_counter, claim the next ID
3. Write updated counter + registry entry
4. Commit and push
5. If push rejected (someone else registered first), pull and retry

Returns the assigned node_id on success.