refactor: split config into remotes/virtuals/locals sections

Repository types now live under dedicated top-level keys instead of a
shared remotes: block distinguished by a type field:

  remotes:   caching proxy remotes (no type field needed)
  virtuals:  virtual merged-index repositories
  locals:    local upload repositories

Routes for local repos move from /api/v1/remote/ to /api/v1/local/.
config.py gains get_virtual_config() and get_local_config() lookups.
Root endpoint now reports all three sections. Drop root conf.d/ (was
an exact duplicate of examples/conf.d-method/).

tests/test_virtual.py

@@ -430,10 +430,10 @@ class TestVirtualRoute:
         response = client.get("/api/v1/virtual/no-such-virtual/index.yaml")
         assert response.status_code == 404
 
-    def test_non_virtual_type_returns_400(self, client, patched_virtual_deps):
-        # helm-test is type "remote", not "virtual"
+    def test_non_virtual_name_returns_404(self, client, patched_virtual_deps):
+        # helm-test is in remotes, not virtuals
         response = client.get("/api/v1/virtual/helm-test/index.yaml")
-        assert response.status_code == 400
+        assert response.status_code == 404
 
     def test_unsupported_package_returns_400(self, client, patched_virtual_deps):
         # unsupported-virtual-test has package "rpm"