agent: collapse capability As* helpers onto two generic gates

Every As* helper repeated the same nil-check / type-assert / CapabilityDeclarer-gate dance. Fold the logic into declaredCapability[T] (assert + DeclaredCaps gate) and builtinCapability[T] (assert only, for built-in-only capabilities), leaving each As* a one-line wrapper with its existing doc contract. Behavior is unchanged and covered by the existing per-capability gate tests in capabilities_test.go.