fix(mem0-plugin): fix userConfig schema and restore ${user_config.*} in .mcp.json

userConfig was missing required `type` and `title` fields per Claude Code
plugin docs — value was never collected, so ${user_config.MEM0_API_KEY}
could not resolve. Added both fields. Restored .mcp.json to use
${user_config.MEM0_API_KEY} for the MCP server Authorization header.

This completes the fix for #4876: plugin now prompts for API key at
install, stores in keychain, and MCP server reads it via user_config
interpolation — no shell env var needed.

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

@@ -12,6 +12,8 @@
   "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

mem0-plugin/.mcp.json

@@ -4,7 +4,7 @@
       "type": "http",
       "url": "https://mcp.mem0.ai/mcp/",
       "headers": {
-        "Authorization": "Token ${MEM0_API_KEY}"
+        "Authorization": "Token ${user_config.MEM0_API_KEY}"
       }
     }
   }