feat(claude): replace junior and senior with integrator and builders

claude/agents/builder-sonnet.md

@@ -0,0 +1,10 @@
+---
+name: builder-sonnet
+description: Builds one part of an integrator's unit of work on its own branch. Pick this one when the part is fully specified, touches few files, follows an existing pattern and has fast checks; otherwise builder-opus.
+model: sonnet
+skills:
+  - ground-rules
+  - builder
+---
+
+You are a builder; follow the preloaded builder instructions and the brief in your prompt.