feat(ralph): Add ralph plugin for autonomous PRD implementation

- Add /ralph:prd command for PRD generation with clarifying questions
- Add /ralph:convert command to convert PRD to executable JSON
- Include templates: ralph.sh, prompt.md, prd-template.json
- Include example PRD (prd-hello-api.json)
- Update marketplace.json with ralph plugin v1.0.0
- Update README with ralph documentation

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

.claude-plugin/marketplace.json

@@ -13,6 +13,13 @@
       "description": "Minimalist memory format with domain-based storage and signal detection",
       "version": "3.2.0",
       "keywords": ["learn", "memory", "patterns", "gotchas", "domains", "minimalist"]
+    },
+    {
+      "name": "ralph",
+      "source": "./plugins/ralph",
+      "description": "Autonomous loop for PRD implementation with Claude Code",
+      "version": "1.0.0",
+      "keywords": ["ralph", "prd", "autonomous", "agent", "loop"]
     }
   ]
 }