Feature request: Allow disabling built-in MCP servers per-project

Problem

Built-in MCP servers (Figma, Playwright, html_to_design) load in every Claude Code session regardless of project context. This wastes startup time and consumes context tokens (~60 tool definitions) even when they're not needed.

Current behavior

• disabledMcpjsonServers only works on user-defined .mcp.json servers
• Permission deny lists block tool calls but tools still load and consume tokens
• --disallowedTools flag has the same limitation
• There is no way to prevent built-in servers from loading their tool definitions

Proposed solution

Add a setting like disabledBuiltinServers (or extend disabledMcpjsonServers) that prevents built-in MCP servers from loading at all in specific projects:

// In .claude/settings.json (project-level)
{
  "disabledBuiltinServers": ["claude_ai_Figma", "claude_ai_html_to_design", "figma", "playwright"]
}

This would allow users to scope MCP servers to only load where they're actually needed, reducing token usage and improving startup time.

Workaround

Setting ENABLE_TOOL_SEARCH to a lower threshold helps by deferring tool definitions, but the servers still start and the tools still exist in the deferred pool.