[Feature Request] Add project-exclusive MCP configuration option to disable user-level aggregation

Title: Support project-exclusive MCP configuration (disable user-level MCP aggregation per project)

Problem:

When working across multiple projects with different MCP requirements, user-level MCPs from ~/.claude.json are always aggregated with project-level .mcp.json configs. This causes issues when:

• Projects have isolated environments (e.g., project-a with MongoDB servers vs project-b with PostgreSQL servers)
• Loading irrelevant MCPs adds noise and potential confusion
• Security/access concerns when one project's database MCPs shouldn't be available in another project's context

Current Behavior:

Starting Claude Code from any directory always loads:

1. User-level MCPs (~/.claude.json)
2. Project-level MCPs (.mcp.json)

No way to opt out of user-level aggregation at the project level.

Desired Behavior:

Add an option in project .mcp.json to disable user-level MCP aggregation:

{
"mcpServers": { ... },
"exclusive": true
}

When "exclusive": true, only that project's .mcp.json servers would load, ignoring ~/.claude.json MCPs.

Use Case:

Working in a workspace with multiple sub-projects:

• /workspace/.mcp.json - aggregate all MCPs (default behavior)
• /workspace/project-a/.mcp.json - exclusive, only project-a database servers
• /workspace/project-b/.mcp.json - exclusive, only project-b database servers

This enables clean context isolation per project while preserving aggregation when desired.