Add disabled/enabled toggle for MCP servers

Feature Request

Add an enabled: false (or disabled: true) flag for MCP server configuration, so servers can remain in the config but not auto-load on startup.

Current Behavior

MCP servers are either configured (and auto-load every session) or not configured at all. There's no middle ground — if you want to stop a server from loading, you must claude mcp remove it entirely and re-add it later with all its configuration.

Desired Behavior

Allow something like:

{
  "mcpServers": {
    "canva": {
      "command": "...",
      "enabled": false
    }
  }
}

• Servers with enabled: false stay in the config but don't auto-connect on startup
• Users can toggle them on/off via /mcp dialog or claude mcp enable <name> / claude mcp disable <name>
• This preserves all server configuration (command, args, env vars, auth tokens) between enable/disable cycles

Why This Matters

• Users with many MCP servers (10+) experience context bloat — each server adds deferred tools to every conversation
• Removing and re-adding servers is friction-heavy, especially for cloud/OAuth MCPs that require re-authentication
• Most users only need a subset of their servers in any given session
• This is a common pattern in other tools (VS Code extensions, browser extensions, etc.)

Suggested UX

• claude mcp disable <name> — keeps config, stops auto-loading
• claude mcp enable <name> — resumes auto-loading
• /mcp dialog shows disabled servers with a toggle to re-enable