plugin: restructure to marketplace + plugins/<name> layout

Source-of-truth claude/ now matches the proven shape used by
anthropics/claude-plugins-official and other shipping marketplaces:

  claude/.claude-plugin/marketplace.json    references ./plugins/fj
  claude/plugins/fj/.claude-plugin/plugin.json
  claude/plugins/fj/skills/fj/SKILL.md

Previous string-source "./" and structured {source: url} forms both
tripped "source type your Claude Code version does not support."
The relative sub-path form works.

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

claude/.claude-plugin/marketplace.json

@@ -13,10 +13,7 @@
     {
       "name": "fj",
       "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).",
-      "source": {
-        "source": "url",
-        "url": "https://github.com/rasterandstate/fj-claude-plugin.git"
-      },
+      "source": "./plugins/fj",
       "license": "MIT",
       "category": "developer-tools",
       "keywords": ["fj", "forgejo", "gitea", "git", "cli"],

claude/README.md

@@ -25,15 +25,22 @@ After install, the skill activates automatically when relevant.
 ```
 claude/
 ├── .claude-plugin/
-│   ├── plugin.json         plugin manifest (name, version, keywords)
-│   └── marketplace.json    makes this directory a single-plugin
-│                           marketplace; required for `/plugin install`
+│   └── marketplace.json    marketplace manifest — declares plugins[]
 ├── README.md               this file
-└── skills/
+└── plugins/
     └── fj/
-        └── SKILL.md        the skill body — what Claude reads
+        ├── .claude-plugin/
+        │   └── plugin.json plugin manifest (name, version, keywords)
+        └── skills/
+            └── fj/
+                └── SKILL.md  the skill body — what Claude reads
 ```
 
+The marketplace lives at the root; each plugin lives in `plugins/<name>/`.
+`marketplace.json` references plugins via `"source": "./plugins/<name>"`.
+This is the working pattern used by `anthropics/claude-plugins-official`
+and other shipping marketplaces.
+
 ## Updating
 
 This `claude/` directory is the source of truth, inside the fj repo at