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>

.claude-plugin/plugin.json

@@ -0,0 +1,10 @@
+{
+  "name": "tea",
+  "description": "Gitea CLI (tea) reference plus a mandatory-login guard. Ships /tea:login (pin a login) and /tea:use (command reference), and a PreToolUse hook that blocks any tea command that would touch Gitea without --login.",
+  "version": "1.0.0",
+  "author": {
+    "name": "naudachu"
+  },
+  "license": "MIT",
+  "keywords": ["gitea", "tea", "cli", "git", "login-guard"]
+}