1. Cook pasta, uncovered, in 3 to 4 quarts boiling water until barely tender to bite, 5 to 7 minutes.
2. Meanwhile, stem and seed bell peppers and cut lengthwise into 1/4-inch slices. In a 10- to 12-inch nonstick frying pan over medium-high heat, frequently stir peppers and oil until peppers are limp and browned, 12 to 14 minutes; reduce heat if peppers start to darken too much.
3. Cut chicken into 1/4- by 2-inch strips. When pasta is barely tender to bite, add chicken to pasta water and cook until meat is no longer pink in center (cut to test), 2 to 3 more minutes. Drain mixture and return to pan.
4. Blend yogurt with cornstarch. Add mixture and taco sauce to peppers; stir often over medium heat until bubbling, 3 to 5 minutes.
5. Add the pepper-yogurt sauce and chopped cilantro to the pasta and chicken; mix to coat.