…nterprise consumer

Patches the composite action so the upcoming loft-enterprise Step 2 consumer PR
(ENGQA-1042) can drop the inline setup block in prerelease-checks.yaml without
re-introducing workarounds:

- AWS Login: set output-credentials: true so consumer steps (run-ginkgo) can read
  AWS_ACCESS_KEY_ID / AWS_SECRET_ACCESS_KEY / AWS_SESSION_TOKEN via env.AWS_*.
- Resolve step: add "empty -> latest pre-release" resolver for
  standalone-vcluster-upgrade-version, and "empty -> latest stable" resolver for
  platform-base-version. Both inputs are now optional with the same fallback
  semantics the pre-extraction inline workflow had.
- Resolve step: mirror resolved versions to $GITHUB_ENV
  (STANDALONE_VCLUSTER_VERSION, STANDALONE_VCLUSTER_UPGRADE_VERSION,
  PLATFORM_BASE_VERSION, PLATFORM_RC_VERSION, DEFAULT_VCLUSTER_CHART_VERSION)
  alongside the existing $GITHUB_OUTPUT writes. Downstream steps that read these
  as env vars (Summarize, Run pre-release ...) keep working unchanged.
- New "Verify release assets" step: HEADs the install-standalone.sh (base + upgrade)
  and vcluster-linux-amd64 (upgrade) URLs and fails fast before downstream EC2 /
  VCI provisioning if a release is missing assets.

README updated to match. Tag as prerelease-setup/v1.1 after merge.

…EAD, semver guard)

- Remove output-credentials: true. configure-aws-credentials exports
  AWS_ACCESS_KEY_ID / AWS_SECRET_ACCESS_KEY / AWS_SESSION_TOKEN to
  $GITHUB_ENV by default; output-credentials only controls additional
  step-output exposure (steps.X.outputs.aws-session-token) and has
  nothing to do with the env-var path the consumer actually uses. The
  previous reasoning in the code comment was wrong; both flag and
  comment are dropped.
- Verify release assets: use HEAD (curl -sfIL) instead of GET so the
  ~50 MB vcluster-linux-amd64 body is not downloaded just to probe the
  URL.
- Resolve step: reject an older-branch pre-release as the upgrade
  target (e.g. base=0.34.1, latest pre-release=0.33.5-rc.1). sort -V
  puts the higher semver last; the upgrade input must equal that tail.
  Apply the same guard to RC vs BASE on the platform side — same bug
  class, same fix.
- README + action description updated to match.