Adds a mirror-it skill that registers a Gitea push mirror via `tea api`
against the push_mirrors endpoint, using Gitea's actual
remote_address/remote_username/remote_password fields (never embedding
credentials in the mirror URL). Credentials can come from the
environment or fall back to a repo-root .env file, and all shell
snippets are POSIX-compatible so the skill isn't tied to any one shell.
Also ignores .env* files repo-wide.
