Improve MCP experience

## Summary

Improve the overall MCP (Model Context Protocol) experience in Mux with better tooling and configuration options.

## Areas for Improvement

### 1. Detecting config issues
- Surface configuration problems early and clearly
- Provide actionable error messages when MCP servers fail to connect or have misconfigured settings
- Validate MCP configuration on startup/reload

### 2. Process management
- Better visibility into MCP server process status
- Graceful handling of MCP server crashes/restarts
- Tools for debugging MCP server lifecycle issues

### 3. Configuring for Mux, not just per workspace
- Support global/Mux-level MCP configuration in addition to per-workspace config
- Allow users to define default MCP servers that apply across all workspaces
- Reduce duplication when the same MCP servers are needed in multiple workspaces

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Created on behalf of @ibetitsmike