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

devenv.nix

@@ -1,4 +1,5 @@
 {
+  pkgs,
   ...
 }:
 {
@@ -9,8 +10,17 @@
     };
   };
 
+  packages = [
+    pkgs.basedpyright
+  ];
+
   git-hooks.hooks = {
-    pyright.enable = true;
+    basedpyright = {
+      enable = true;
+      entry = "${pkgs.basedpyright}/bin/basedpyright";
+      files = "\\.py$";
+      types = [ "file" ];
+    };
     ruff.enable = true;
     ruff-format.enable = true;
     commitizen.enable = true;