Charcoal ashes in garden?

If the charcoal was thoroughly burned, there is not likely to be any starter residue. The starter material (fluid or solid) is generally fairly volatile (which is what helps it start the fire), so it would probably be gone at the beginning of the cooking sequence. After all, you don't want the starter in the food you're cooking, so you have to wait for the fire to get really hot. If the fire were put out prematurely (before it got hot enough to cook anything) there may be some residue. Look at the ash to see if there is a lot of unburned charcoal in it.

Wood ash has a high pH, which is sometimes used for amending acid soils, but charcoal ash generally has a pH a little lower (I think this is due to the higher temperature that charcoal burns at). Plants need lots of nutrients, so if there is some soil, compost, etc. mixed with the ash (so that it's not predominantly charcoal ash), there shouldn't be a problem growing things. The charcoal is basically made from wood, so there shouldn't be any toxicity issues. Once again, the charcoal is used to barbeque things, so you don't want toxic substances in it. Your garden is one step removed from the cooking process, so the toxicity problems will be lower there then in the barbequeing.

Doug Kanter wrote:

Kingsford briquettes contain limestone dust, which may cause pH problems for the plants.

Fluid won't be a problem, unless it was poured on the soil.