feat: add mirror-it plugin for Gitea to GitHub push mirroring

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.

dist/agy/mirror-it/plugin.json

@@ -0,0 +1,3 @@
+{
+  "name": "mirror-it"
+}