fix!: drop the always-empty return value from Create

[tas50]

Breaking change. Flagged fix!; happy to hold this one if you'd rather not spend the break.

Nodes.Create, Roles.Create and Environments.Create returned a *T decoded from the POST response body. The Chef Server answers those endpoints with {"uri":…}, not the created object — so the returned value was always a zero-valued struct behind a non-nil pointer.

The repo's own fixture says as much (roles_test.go:16):

srv.Handle("POST /organizations/o/roles",
    cinctest.Route{Status: 201, Body: `{"uri":"http://x/roles/db"}`})

and every call site in the repo already discarded the value. Confirmed against cinc-zero too: Nodes.Create returns &Node{} with every field empty.

The change

node, resp, err := c.Nodes.Create(ctx, n)   // before — node is always &Node{}
resp, err := c.Nodes.Create(ctx, n)         // after

Now matches Delete, which already returns (*Response, error).

Clients.Create deliberately keeps its *APIClient: chef_key genuinely is in that response, and it carries the generated private key a caller has exactly one chance to capture.

Migration

Mechanical — drop one return value at each call site:

- if _, _, err := c.Nodes.Create(ctx, n); err != nil {
+ if _, err := c.Nodes.Create(ctx, n); err != nil {

Test plan

• TestNodesCreate_ReturnsOnlyResponseAndError — asserts the 201 comes back on *Response (did not compile before the change, which is the red for a signature change)
• All in-repo callers updated: nodes_test.go, roles_test.go, environments_test.go, integration/integration_test.go
• go vet ./... clean
• go test ./... -race -count=2
• cd integration && go test ./...
• README Status table unchanged — no service, method, option or type is added, removed or renamed; the table does not carry signatures