[Feature]: `reusable-ci-nightly-benchmark.yaml` resets llm-d to origin/main, so changes in an llm-d PR can't be /test-validated before merge

[aavarghese]

Problem Statement

Summary

When the nightly benchmark workflow is triggered against an llm-d PR (via /test / workflow_dispatch), the Checkout Code step correctly checks out the PR ref — but a later step (Conditionally clone llm-d from main repo) immediately does git reset --hard origin/main on that same checkout, discarding the PR's changes. As a result the run executes against origin/main's guide content, not the PR's, so any change under guides/** (manifests, kustomize overlays, guide.yaml, README) is silently reverted and never exercised.

Root cause

.github/workflows/reusable-ci-nightly-benchmark.yaml

if [[ -d ~/_work/llm-d/llm-d ]]; then
  ln -s ~/_work ~/work        # symlink → same tree the PR was checked out into
fi
if [[ ! -d ~/work/llm-d ]]; then
  git clone https://github.com/llm-d/llm-d.git
else
  # Refresh the persistent checkout so it doesn't run against stale guide config.
  git -C ~/work/llm-d/llm-d fetch --prune origin
  git -C ~/work/llm-d/llm-d reset --hard origin/main   # ← discards the PR checkout
  ...
fi

On the persistent self-hosted runner, ~/work/llm-d/llm-d is the PR checkout (via the ~/_work → ~/work symlink), so reset --hard origin/main overwrites the PR's tree with main.

Impact

The reset --hard origin/main discards the entire checked-out PR tree, but the nightly only consumes guides/** from that checkout at runtime (kustomize repoPath, guide manifests/overlays/RBAC/router values/env.sh). So any llm-d PR changing guide content cannot be validated by this nightly pre-merge — it always runs against origin/main's guides

Evidence

https://github.com/llm-d/llm-d/actions/runs/30385519489

Proposed Solution

Proposed fix

Make the refresh target the ref under test instead of hardcoding origin/main. Options:

1. When triggered for a PR (github.event_name / a passed head SHA/ref), reset --hard to that ref rather than origin/main; only fall back to origin/main for scheduled runs.

Alternatives Considered

No response

Willingness to Contribute

Yes, I can submit a PR

Additional Context

No response