Look up a configuration value by key, with an optional default. The config
object is whatever was passed to the skill’s constructor (typed as
SkillConfig, a string-keyed record). Values are returned as-is; the caller
provides the type argument for casting.
The configuration key to look up.
Value to return when the key is not present. The result is cast to T.
T — the configured value cast to the generic type, or the default value.