chore(mcp): allow mcp-server-darwin-arm64 in validation allowlist (#1107)

Local pkg-helper installs an arm64 mcp-server binary on macOS hosts; the
existing allowlist (deno, bun, uvx, ...) didn't cover it so MCP server
config validation rejected legitimate entries.

internal/mcp/validation.go

@@ -21,6 +21,7 @@ var allowedCommands = map[string]bool{
 	"java": true, "dotnet": true, "php": true,
 	"uvx": true, "uv": true, "pipx": true,
 	"deno": true, "bun": true,
+	"mcp-server-darwin-arm64": true,
 }
 
 // Shell metacharacters that indicate injection attempt.