chore: clean workspace deps and add CLAUDE.md

- Drop ipykernel from the root virtual dependency list; the
  workspace relies on its subpackages and uv lockfile to manage
  per-service dependencies.
- Refresh uv.lock to match the trimmed root dependency set.
- Add CLAUDE.md describing the workspace layout, common
  development and database commands, the Nginx/backend/runtime
  auth request flow, and the boundaries to keep in mind when
  changing services.

uv.lock

@@ -1079,12 +1079,6 @@ wheels = [
 name = "model-platform"
 version = "0.1.0"
 source = { virtual = "." }
-dependencies = [
-    { name = "ipykernel" },
-]
-
-[package.metadata]
-requires-dist = [{ name = "ipykernel", specifier = ">=7.3.0" }]
 
 [[package]]
 name = "nbclient"