fix(client): surface nested error bodies and implement --debug

[vks-team]

Summary

While running grn vks generate-kubeconfig, a real server validation error was hidden behind a bare API error (HTTP 400 Bad Request). Two client deficiencies caused this:

1. Nested error bodies were dropped. The VKS API returns {"error":{"message":...}} (a nested object). formatError only extracted error when it was a string, so the message was lost. Added a raw-body fallback so the server's message is always surfaced.
2. --debug was a no-op. The debug flag was stored but never used. Now logs request method/URL/body and response status/body to stderr.

With the fix, the same call shows the actual reason:

[debug] response 400: {"error":{"message":"KubeConfig can only be requested when the cluster is ACTIVE, UPDATING, or WAITING_UPDATE."}}

Test Plan

• go test ./internal/client/ — new TestFormatErrorSurfacesNestedErrorObject, TestFormatErrorUsesPlainStringMessage, existing tests PASS
• go vet ./... clean; build clean
• Manual: grn vks generate-kubeconfig --debug now prints request/response and the nested error message

Tests on macOS 26 + Go 1.22.2 need the external linker: CGO_ENABLED=1 go test -ldflags='-linkmode=external' ./...

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