refactor: switch to basedpyright, remove pydantic-settings

- Replace pyright with basedpyright in devenv.nix (custom hook)
- Add basedpyright to devenv packages
- Fix all basedpyright warnings: add DiGraph[str] type args, annotate
  class attributes, narrow SyncResponse, handle unused call results,
  suppress unavoidable Any from yaml.safe_load and untyped blackforest
- Replace pydantic-settings[yaml] with direct pyyaml dependency
- Update CLAUDE.md to reflect basedpyright and dependency changes

pyrightconfig.json

@@ -1,4 +1,4 @@
 {
   "venvPath": ".",
-  "venv": ".devenv/state/venv"
+  "venv": ".devenv/state/venv",
 }