fix(mem0-plugin): remove userConfig from plugin.json, add auto-deps install

- Remove userConfig from .claude-plugin/plugin.json — OAuth flow handles
  auth via mcp.mem0.ai, userConfig prompt was redundant and confusing
- Add ensure_deps.sh to install mem0ai SDK into persistent plugin venv
- Add requirements.txt (mem0ai) for dependency tracking
- Wire SessionStart hook to auto-install deps on first run
- Update setup_coding_categories.py to find venv site-packages
- Update onboarding SKILL.md with venv-aware install steps

mem0-plugin/.claude-plugin/plugin.json

@@ -9,14 +9,5 @@
   "homepage": "https://mem0.ai",
   "repository": "https://github.com/mem0ai/mem0",
   "license": "Apache-2.0",
-  "keywords": ["memory", "personalization", "mcp", "semantic-search"],
-  "userConfig": {
-    "MEM0_API_KEY": {
-      "type": "string",
-      "title": "Mem0 API key",
-      "description": "Your Mem0 API key (get one at https://app.mem0.ai/dashboard/api-keys or run: mem0 init --agent --json)",
-      "required": true,
-      "sensitive": true
-    }
-  }
+  "keywords": ["memory", "personalization", "mcp", "semantic-search"]
 }