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>

skills/auth/SKILL.md

@@ -0,0 +1,51 @@
+---
+name: auth
+description: Pin the Gitea login used by the tea CLI in this project. Run when the tea-guard hook reports no login is pinned, or when the user types /tea:auth. Enumerates available logins, makes the OPERATOR pick one, and persists it to .claude/settings.local.json. The pin takes effect immediately — no restart.
+---
+
+# /tea:auth — pin the project Gitea login
+
+Goal: have the **operator** select exactly one `tea` login for this project and
+persist it to `.claude/settings.local.json` under `env.GITEA_LOGIN`. The
+`tea-guard` hook reads this file at call time and rewrites every
+`--login "$GITEA_LOGIN"` to the pinned value, so the choice takes effect
+**immediately, with no session restart**.
+
+## The one hard rule: the operator chooses, never you
+
+Picking the wrong identity is the exact failure this command exists to prevent.
+So:
+
+- **ALWAYS** present the choice with `AskUserQuestion` and let the operator
+  pick — even if memory, context, the repo URL, or a previous session suggests
+  a "likely" login. Do **not** auto-select from memory or infer it. A wrong
+  guess writes under the wrong account.
+- The only exception: exactly **one** login exists on the machine — then
+  propose it and still confirm before writing.
+
+## Steps
+
+1. Enumerate logins (allowed by the guard even with no pin):
+   `tea logins list -o json`
+2. **No logins:** stop and ask the operator to run `tea logins add` themselves
+   — it is interactive (prompts for URL/token). Do not run it for them.
+3. **One login:** propose it; confirm before writing.
+4. **Several logins:** `AskUserQuestion` with each login's `name`, `user`, and
+   `url` so the operator's choice is unambiguous. Never decide for them.
+5. Merge the chosen name into `.claude/settings.local.json` under `env`
+   (do not clobber other keys):
+   ```json
+   { "env": { "GITEA_LOGIN": "<chosen-name>" } }
+   ```
+6. Done — it is live. The guard resolves the pin from the file on the next
+   `tea` call; no restart needed. Tell the operator which login is now pinned.
+
+## Identity-safety rules
+
+- NEVER run commands that mutate logins or global login state:
+  `tea logins add/edit/delete/default`, `tea logout`. Read-only
+  `tea logins list` is the only allowed login command.
+- If a `tea` call fails with a permission/scope error, report it. Do NOT try to
+  fix it by switching to, or editing, a different login.
+- If you ever see `no gitea login detected, falling back to login '...'`, treat
+  it as a hard failure: stop, do not act on the result, surface it.