fix(plugin): move marketplace.json to repo root so GitHub install works

Follow-up to #374. The nested location
(packages/plugin/.claude-plugin/marketplace.json) still prevented the
GitHub-URL install flow because the CLI always looks for
'<repo-root>/.claude-plugin/marketplace.json' — '--sparse' limits
checkout scope but does not change the manifest lookup path.

Evidence of the remaining bug after #374:

  $ claude plugin marketplace add https://github.com/toss/react-simplikit \
      --sparse packages/plugin
  ✘ Failed to add marketplace: Marketplace file not found at
    <temp>/.claude-plugin/marketplace.json

Changes:
- Create .claude-plugin/marketplace.json at the repo root with
  'source': './packages/plugin' pointing at the plugin
- Delete the redundant packages/plugin/.claude-plugin/marketplace.json
- Update the README install command to sparse-checkout both the
  root manifest and the plugin directory:
    --sparse .claude-plugin packages/plugin

Verified locally (using the repo-root path, which mirrors what the
clone looks like after merge):

  claude plugin validate .                               → ✓ Passed
  claude plugin marketplace add <repo-root> --scope local → ✓ Added
  claude plugin install react-design-philosophy@...       → ✓ Installed, enabled

Author: kimyouknow

…s/plugin/.claude-plugin/marketplace.json .claude-plugin/marketplace.jsonpackages/plugin/.claude-plugin/marketplace.json renamed to .claude-plugin/marketplace.json packages/plugin/.claude-plugin/marketplace.json renamed to .claude-plugin/marketplace.json

@@ -7,7 +7,7 @@
   "plugins": [
     {
       "name": "react-design-philosophy",
-      "source": "./"
+      "source": "./packages/plugin"
     }
   ]
 }

packages/plugin/README.md

@@ -7,7 +7,7 @@ React design philosophy plugin for Claude Code. Includes skills for reviewing an
 ```bash
 # 1. Add this plugin's marketplace (sparse-checkout keeps the clone minimal)
 claude plugin marketplace add https://github.com/toss/react-simplikit \
-  --sparse packages/plugin
+  --sparse .claude-plugin packages/plugin
 
 # 2. Install the plugin
 claude plugin install react-design-philosophy@react-design-philosophy