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/use/references/tea/index.md

@@ -0,0 +1,36 @@
+# tea CLI — Index
+
+Version: `tea 0.14.1` (go-sdk v0.25.1). Source: recursive `--help` traversal. Upstream: https://gitea.com/gitea/tea
+
+`tea` is a productivity helper for Gitea. It uses the current git repository context (`$PWD`) — owner/repo/login are auto-detected when inside a repo. Config is persisted in `$XDG_CONFIG_HOME/tea`.
+
+## Global options
+
+- `--debug, --vvv` — enable debug mode
+- `--help, -h`, `--version, -v`
+
+## Common flags (present on nearly every command)
+
+| Flag | Purpose |
+|---|---|
+| `--login, -l <name>` | use a specific login from the config |
+| `--repo, -r <owner/repo>` | override repository context (local path or slug) |
+| `--remote, -R <name>` | discover login from this git remote |
+| `--output, -o <fmt>` | output format: `simple, table, csv, tsv, yaml, json` |
+| `--page, -p <n>` / `--limit, --lm <n>` | pagination (defaults 1 / 30) |
+| `--fields, -f <list>` | which columns to print |
+
+## Command categories
+
+```
+ENTITIES:   issues, pulls, labels, milestones, releases, times,
+            organizations, repos, branches, actions, webhooks, comment
+HELPERS:    open, notifications, clone, api
+MISC:       whoami, admin
+SETUP:      logins, logout, ssh-keys
+```
+
+- [ENTITIES](./entities.md)
+- [HELPERS](./helpers.md)
+- [MISC](./misc.md)
+- [SETUP](./setup.md)