create_ssh_key.sh: self-hosted Host block uses alias only, so remotes on the real hostname skip IdentityFile

[ulises-c]

Summary

For a self-hosted Git server, SSH_and_GPG/create_ssh_key.sh writes an SSH config
block whose Host line contains only the alias, while HostName holds the real
server hostname. As a result, any git remote that is cloned/configured with the
real hostname (rather than the alias) never matches the block, so the block's
IdentityFile is not applied. SSH then falls back to whatever default keys the
agent offers, and authentication fails with Permission denied (publickey) even
though the correct key exists on disk and is enrolled on the server.

Root cause

SSH_and_GPG/create_ssh_key.sh, self-hosted branch (around lines 159–168):

if [[ -n "$IS_SELF_HOSTED" ]]; then
  {
    echo ""
    echo "Host $GIT_HOST"            # <-- alias only
    echo "  HostName $GIT_HOSTNAME"  # <-- real hostname lives here
    ...
    echo "  IdentityFile $KEY_PATH"
  } >> "$CFG_PATH"

SSH matches config blocks against the connection target. Git remotes generated
against a self-hosted server typically use the real hostname in the URL
(ssh://git@<real-host>/...), not the alias. Since the Host line lists only the
alias, that connection doesn't match, the IdentityFile is skipped, and the wrong
key is offered.

Impact

• git fetch/push/clone and git submodule update against the self-hosted
  server fail with Permission denied (publickey).
• It looks like a key or server problem, but the key is fine — only the config
  match is wrong. Connecting via the alias (ssh <alias>) succeeds while connecting
  via the real hostname fails, which is the tell.

Reproduction (sanitized)

1. Run the script, choose "Self-Hosted Git Server", give an alias and a distinct
   real hostname.
2. Add a remote using the real hostname:
   git remote add origin ssh://git@<real-host>/<owner>/<repo>.git
3. ssh -T <alias> → authenticates.
   ssh -T git@<real-host> → Permission denied (publickey).

Suggested fix

Include both the alias and the real hostname on the Host line so either target
matches the block:

echo "Host $GIT_HOST $GIT_HOSTNAME"

(When alias and hostname are equal, de-dupe so the token isn't repeated.)

Secondary note

Unlike add_remote_host.sh, the self-hosted block in create_ssh_key.sh omits
IdentitiesOnly yes. Adding it would keep the agent from offering unrelated keys
first and makes the intended key deterministic. Worth aligning the two scripts.