Feature Request: disabled option for MCP servers in .mcp.json

Problem

All configured MCP servers load their tool schemas into context at session start, consuming tokens even when unused.

Users must choose between:

• Keep servers configured → waste context on unused tools
• Remove servers → lose access when needed

Proposed Solution

Add disabled option to MCP server config:

{
  "mcpServers": {
    "serena": {
      "type": "stdio",
      "command": "uvx",
      "args": ["--from", "git+https://github.com/oraios/serena", "serena-mcp-server"],
      "disabled": true
    },
    "deepwiki": {
      "type": "http",
      "url": "https://mcp.deepwiki.com/mcp"
    }
  }
}

Expected Behavior

• disabled: true → Server not connected, tools not loaded into context
• disabled: false or omitted → Current behavior (connected at session start)
• Optional enhancement: Enable at runtime via /mcp enable serena

Benefits

• Keep configurations without context overhead
• Easy toggle without claude mcp remove/add
• Similar pattern to VS Code extension disable

Use Case

Power users with 5+ MCP servers configured only need 1-2 per session. Currently forced to remove unused servers or accept ~3000+ tokens of tool schemas loaded every session.