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."
+    }
+  ]
+}

README.md

@@ -19,21 +19,29 @@ A Claude Code plugin that gives Claude a reference for the `tea` CLI and enforce
 
 ## Installation
 
-Clone this repo into your Claude Code plugins directory:
+This is a Claude Code plugin — install it through the plugin marketplace, not by hand-editing `settings.json`.
 
-```bash
-git clone <repo-url> ~/.claude/plugins/tea
-```
+1. Register this repo as a marketplace:
 
-Then add it to your project's `.claude/settings.json`:
+   ```
+   /plugin marketplace add https://git.noodles.cam/claude-skills/tea.git
+   ```
 
-```json
-{
-  "plugins": ["~/.claude/plugins/tea"]
-}
-```
+   Already have a local clone? Point at the directory instead:
 
-Or to your global settings (`~/.claude/settings.json`) to enable it everywhere.
+   ```
+   /plugin marketplace add /path/to/tea
+   ```
+
+2. Install the plugin:
+
+   ```
+   /plugin install tea@tea
+   ```
+
+The skills (`/tea:login`, `/tea:use`) and the `tea-guard` hook load immediately. Use `/plugin` to enable, disable, or update it later.
+
+> The marketplace registration is written to `extraKnownMarketplaces` and the plugin to `enabledPlugins` in your settings automatically — you don't edit those by hand. There is **no** top-level `"plugins"` settings key; if you've added one from older instructions, remove it.
 
 ## First use
 
@@ -61,7 +69,9 @@ This prevents silent fallback to the machine's default login (often a personal a
 ## Project layout
 
 ```
-.claude-plugin/plugin.json   plugin manifest
+.claude-plugin/
+  plugin.json                plugin manifest
+  marketplace.json           marketplace catalog (makes `/plugin install` work)
 hooks/
   hooks.json                 registers the PreToolUse hook
   tea-guard.sh               the guard (Python 3, no deps)