readme: fix install via plugin marketplace

The install steps used a non-existent "plugins" settings key, so a fresh
clone loaded nothing. Add .claude-plugin/marketplace.json and document the
supported /plugin marketplace add + /plugin install tea@tea flow.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

.claude-plugin/marketplace.json

@@ -0,0 +1,13 @@
+{
+  "name": "tea",
+  "owner": {
+    "name": "naudachu"
+  },
+  "plugins": [
+    {
+      "name": "tea",
+      "source": "./",
+      "description": "Gitea CLI (tea) reference plus a mandatory-login guard. Ships /tea:login, /tea:use, and a PreToolUse hook that blocks any tea command that would touch Gitea without --login."
+    }
+  ]
+}