aither-pack-authoring

Build a tool pack for awdk agents: scaffold it with adk pack new, write a tool the model can call, then validate, load-test and bundle it with adk pack validate, adk pack dev and adk pack build. Explains the .toolpack.yaml fields the loader actually reads, the PKA001-PKA009 checks, the reserved namespaces a community pack may not claim, and why validate imports nothing while dev does.