refactor: rename login skill to auth

`/tea:login` could not be invoked as `/login` because Claude Code
reserves that name as a built-in command. Renaming the skill to `auth`
frees the short form: `/tea:auth` and `/auth` both resolve.

Updates all references in the guard hook, README, plugin manifests, and
the use skill.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

.gitignore

@@ -2,3 +2,5 @@
 .docs/
 .claude/
 tmp/
+__pycache__/
+*.pyc