[Bug] /doctor validates individual MCP config files instead of merged result

Bug Description
Title: /doctor should validate merged MCP config, not individual files

Body:

On Windows, I have .mcp.json (checked in, targeting macOS/WSL) with a bare npx command for the Playwright MCP server,
and .mcp.local.json (gitignored) that overrides it with the proper cmd /c npx wrapper for Windows.

At runtime the merge works correctly — the local override takes precedence and the server starts fine. However, /doctor
still flags the checked-in .mcp.json entry as broken because it validates each file independently rather than the merged
result.

Expected behavior: /doctor diagnostics should validate the final merged MCP config (.mcp.json + .mcp.local.json) since
that's what actually runs.

Current behavior: /doctor flags entries in .mcp.json that are overridden by .mcp.local.json, producing false positive
diagnostics on platforms where the checked-in config doesn't apply.

Environment Info

• Platform: win32
• Terminal: windows-terminal
• Version: 2.1.87
• Feedback ID: 46f8b687-bff4-4aaf-a030-f1a8ea19356b

Errors

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