Feature request: Global disabledMcpServers support (user-level, not just per-project)

Summary

The /mcp command allows disabling MCP servers, but the disabled state is stored per-project (under projects[path].disabledMcpServers in ~/.claude.json). There is no way to globally disable an MCP server across all projects.

Problem

Users who have many MCP servers configured (e.g., google-workspace, chrome-devtools, gcloud, proxyman, etc.) but only use a subset regularly must disable the unused ones in every new project individually via /mcp. This is tedious and easy to forget, leading to unnecessary context window usage from tool definitions that are never invoked.

The /doctor command even warns about this:

Context Usage Warnings
  Large MCP tools context (~143,546 tokens > 25,000)

Proposed solution

Support a top-level disabledMcpServers array in ~/.claude.json (alongside the existing mcpServers). The isMcpServerDisabled() function in src/services/mcp/config.ts would check both the global list and the per-project list:

export function isMcpServerDisabled(name: string): boolean {
  const projectConfig = getCurrentProjectConfig()
  if (isDefaultDisabledBuiltin(name)) {
    const enabledServers = projectConfig.enabledMcpServers || []
    return !enabledServers.includes(name)
  }
  // Check global disabled list
  const globalConfig = getGlobalConfig()
  const globalDisabled = globalConfig.disabledMcpServers || []
  if (globalDisabled.includes(name)) return true
  // Check per-project disabled list
  const disabledServers = projectConfig.disabledMcpServers || []
  return disabledServers.includes(name)
}

Per-project settings would override global (a project could re-enable a globally disabled server via its enabledMcpServers list).

Alternatives considered

• Removing servers entirely (claude mcp remove): Loses the configuration. Re-adding later requires remembering the exact command/args/env.
• Managed settings denylist (/Library/Application Support/ClaudeCode/managed-settings.json): Designed for enterprise IT policy, overkill for personal preference. Requires sudo.
• Per-project disabling: The current approach. Works but doesn't scale across many projects.

Additional context

The /mcp UI could add a "scope" option (similar to how claude mcp add has --scope user|project|local) to let users choose whether to disable globally or per-project.