By what reaction does (–)-sparteine decompose in ambient conditions?
Amines left out in the atmosphere will air-oxidize to give N-oxides, which typically have a yellow color. These N-oxides can decompose to give further byproducts. I would store your sparteine under nitrogen or argon atmosphere.

[edit to give examples and mechanism]
The reaction proceeds through a radical chain mechanism with molecular oxygen. Here is one study of the mechanism [from first page of Google search for amines air-oxidation] . 

From wikipedia article for triethylenetetramine. "This oily liquid is colourless but, like many amines, assumes a yellowish color due to impurities resulting from air-oxidation."

"It can be observed that arylamines develop a yellow or black color if left exposed to air for prolonged periods". Ref Note - I realize that sparteine is not an aromatic amine, but the result is the same. 

If you have access to triethylamine or other amines in your lab, do a control experiment: leave one vial open to air, and leave the other under identical conditions except sealed under an inert atmosphere. A few days worth of observation will convince you of the role in oxygen in the yellowing of amines.