Support per-MCP-server startDisabled option

Problem

MCP servers with large tool schemas (e.g., Notion) consume significant tokens on initialization, even in sessions where they aren't needed. Currently, the only options are:

1. Keep the server enabled (always pays the token cost)
2. Remove it from enabledMcpjsonServers (but then it's unavailable via /mcp toggle)

There's no way to have a server available but not auto-initialized.

Proposed Solution

Add a per-server "startDisabled": true option in .mcp.json or settings.local.json:

{
  "mcpServers": {
    "notion-dev": {
      "url": "https://mcp-dev.notion.com/mcp",
      "startDisabled": true
    }
  }
}

Servers with startDisabled: true would:

• Not initialize or load tool schemas on session start
• Appear in /mcp so users can enable them on demand
• Save tokens in sessions where the server isn't needed

Use Case

Our team has a Notion MCP that's only needed ~30% of sessions but consumes a large number of tokens every time. We want it available on demand without paying the cost every session.