psionics are a great idea
I think the “standard” psionic gish is ranger 1 / ardent 4 / slayer
10. That puts you 2 levels behind on manifesting and 1 level behind on attack bonus.
part time skill monkey
For being a substitute skill-monkey and adding a ninja flavor to your character, ask your DM about the option to allow aasimars to qualify for human-only feats, from Races of Destiny, page 150. This seems especially appropriate since you will have the humanoid type and will have spent most of your life as a normal human. Supposing he agrees, you can take the able learner feat from the same page.
Able learner, the skilled city-dweller
alternate class feature, and the build above get you hide, jump, listen, move silently, search, sense motive, spot, and tumble as class skills for all your levels (essentially). That's pretty ninja-ish.