Support agent-scoped MCP tools

Problem

MCP tools are loaded globally and their definitions consume tokens in every conversation, even when the tools are only needed for specific specialized tasks.

For example, iOS simulator MCP tools (~3.2k tokens) are useful when doing UI testing but unnecessary for general coding tasks.

Proposed Solution

Allow MCP servers to be scoped to specific subagent types. Configuration might look like:

{
  "mcpServers": {
    "ios-simulator": {
      "command": "npx",
      "args": ["-y", "@anthropic/ios-simulator-mcp"],
      "scope": ["ios-simulator-agent"]
    }
  }
}

Tools from scoped MCP servers would:

• Not be loaded in the main agent context
• Only be available when a matching subagent is spawned via the Task tool

Benefits

• Reduced token usage in main context
• Cleaner tool namespace for general tasks
• MCP tools still available when needed via specialized agents