Restructure tea skill into a plugin with a mandatory-login guard

Convert the standalone `tea` skill into a skills-dir plugin so commands are
namespaced and an enforcement hook can ship with it:

- /tea:login  — pin the project Gitea login into .claude/settings.local.json
- /tea:use    — tea CLI reference (was the old root SKILL.md), with the
                login rule slimmed since the hook now enforces it
- hooks/tea-guard.sh — PreToolUse(Bash) guard: blocks any `tea` command that
  touches Gitea unless it carries --login and $GITEA_LOGIN is set. Exempts
  `tea logins list` and `tea --version/--help` so /tea:login can bootstrap.

References moved under skills/use/references/. `claude plugin validate` passes;
guard unit-tested across allow/block cases.

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

skills/login/SKILL.md

@@ -0,0 +1,40 @@
+---
+name: login
+description: Pin the Gitea login used by the tea CLI in this project. Run when $GITEA_LOGIN is unset, when the tea-guard hook blocks a command demanding a login, or when the user types /tea:login. Enumerates available logins, lets the user pick one, and persists it to .claude/settings.local.json.
+---
+
+# /tea:login — pin the project Gitea login
+
+Goal: select exactly one `tea` login for this project and persist it to
+`.claude/settings.local.json` under `env.GITEA_LOGIN`, so every later `tea`
+call can pass `--login "$GITEA_LOGIN"`. The `tea-guard` hook blocks every
+Gitea-touching `tea` command until this is done — this command is the
+remediation it points to.
+
+## Steps
+
+1. Enumerate logins (allowed by the guard even without `--login`):
+   `tea logins list -o json`
+2. **No logins:** stop and ask the user to run `tea logins add` themselves —
+   it is interactive (prompts for URL/token). Do not run it for them.
+3. **One login:** propose pinning it; confirm with the user before writing.
+4. **Several logins:** use `AskUserQuestion` to let the user pick. Show each
+   login's `name`, `user`, and `url` so the choice is unambiguous.
+5. Merge the chosen name into `.claude/settings.local.json` under `env` —
+   do not clobber other keys:
+   ```json
+   { "env": { "GITEA_LOGIN": "<chosen-name>" } }
+   ```
+6. Tell the user: the updated `$GITEA_LOGIN` is only exported into Bash after a
+   **session restart**. Until they restart, pass the literal name explicitly:
+   `tea --login <chosen-name> ...`.
+
+## Hard rules (identity safety)
+
+- 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 to the user.
+  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.