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.