plugin: add marketplace.json + correct install instructions

A repo with only plugin.json is recognized as a plugin but not as a
marketplace, which is what `/plugin marketplace add` needs. Adding
.claude-plugin/marketplace.json that declares the marketplace
('rasterandstate') and lists fj as its single plugin.

The correct two-step install is now:

  /plugin marketplace add rasterandstate/fj-claude-plugin
  /plugin install fj@rasterandstate

README.md and claude/README.md updated accordingly.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

README.md

@@ -151,8 +151,9 @@ cargo build --release            # ~4 MB stripped binary at target/release/fj
 plugin so AI agents and developers using Claude Code can drive fj
 directly from natural-language requests.
 
-```sh
+```text
-/plugin install rasterandstate/fj-claude-plugin
+/plugin marketplace add rasterandstate/fj-claude-plugin
+/plugin install fj@rasterandstate
 ```
 
 The source of truth for the plugin lives in [`claude/`](claude/) inside

claude/.claude-plugin/marketplace.json

@@ -0,0 +1,16 @@
+{
+  "name": "rasterandstate",
+  "owner": {
+    "name": "Raster & State",
+    "email": "hello@rasterstate.com",
+    "url": "https://rasterstate.com"
+  },
+  "plugins": [
+    {
+      "name": "fj",
+      "source": ".",
+      "description": "Teach Claude Code to use fj, a CLI for Forgejo and Gitea-compatible instances. Activates when the user mentions fj, Forgejo, Gitea, or any forge-side action (issues, PRs, releases, code search, labels, milestones, webhooks).",
+      "keywords": ["fj", "forgejo", "gitea", "git", "cli"]
+    }
+  ]
+}

claude/README.md

@@ -10,16 +10,12 @@ issues, cut a release, request a review, etc.) on a non-GitHub host.
 
 ## Install
 
-This directory is itself a complete Claude Code plugin. Two ways to
+This directory is both a Claude Code plugin AND a single-plugin
-install:
+marketplace. Add the marketplace once, then install:
 
-```sh
+```text
-# From a local clone of the upstream fj repo:
+/plugin marketplace add rasterandstate/fj-claude-plugin
-/plugin install /path/to/fj/claude
+/plugin install fj@rasterandstate
-
-# From the canonical mirror repo (preferred for users who aren't
-# already cloning fj for development):
-/plugin install rasterandstate/fj-claude-plugin
 ```
 
 After install, the skill activates automatically when relevant.
@@ -29,11 +25,13 @@ After install, the skill activates automatically when relevant.
 ```
 claude/
 ├── .claude-plugin/
-│   └── plugin.json       plugin manifest (name, version, keywords)
+│   ├── plugin.json         plugin manifest (name, version, keywords)
-├── README.md             this file
+│   └── marketplace.json    makes this directory a single-plugin
+│                           marketplace; required for `/plugin install`
+├── README.md               this file
 └── skills/
     └── fj/
-        └── SKILL.md      the skill body — what Claude reads
+        └── SKILL.md        the skill body — what Claude reads
 ```
 
 ## Updating