Release v3.2.1: Add code-reviewer agent to plugin

Fixes #55

Added superpowers:code-reviewer agent to plugin's agents/ directory.
Previously, skills referenced a code-reviewer agent that users needed
to configure personally, causing failures for new users.

Changes:
- New: agents/code-reviewer.md - Agent definition with systematic review checklist
- Updated: skills/requesting-code-review/SKILL.md - Use superpowers:code-reviewer
- Updated: skills/subagent-driven-development/SKILL.md - Use superpowers:code-reviewer
- Version bump: 3.2.0 -> 3.2.1
- Added: .claude-plugin/marketplace.json for local development testing

.claude-plugin/marketplace.json

@@ -0,0 +1,20 @@
+{
+  "name": "superpowers-dev",
+  "description": "Development marketplace for Superpowers core skills library",
+  "owner": {
+    "name": "Jesse Vincent",
+    "email": "jesse@fsck.com"
+  },
+  "plugins": [
+    {
+      "name": "superpowers",
+      "description": "Core skills library for Claude Code: TDD, debugging, collaboration patterns, and proven techniques",
+      "version": "3.2.1",
+      "source": "./",
+      "author": {
+        "name": "Jesse Vincent",
+        "email": "jesse@fsck.com"
+      }
+    }
+  ]
+}

.claude-plugin/plugin.json

@@ -1,7 +1,7 @@
 {
   "name": "superpowers",
   "description": "Core skills library for Claude Code: TDD, debugging, collaboration patterns, and proven techniques",
-  "version": "3.2.0",
+  "version": "3.2.1",
   "author": {
     "name": "Jesse Vincent",
     "email": "jesse@fsck.com"