orchestrate_eval.py crashes when benchmarks_branch is a commit SHA instead of a branch name

Bug

orchestrate_eval.py's create_temporary_branch() function fails with a 404 error when BENCHMARKS_REF is a commit SHA instead of a branch name.

Root Cause

The function resolves the base ref via the GitHub Refs API:

GET https://api.github.com/repos/{repo}/git/refs/heads/{BENCHMARKS_REF}

This endpoint expects a branch name (e.g., main), not a raw commit SHA. When a 40-character SHA is passed, no matching ref exists and the API returns 404.

Reproduction

Trigger an eval with benchmarks_branch set to a commit SHA:

inputs[benchmarks_branch]=641119486f8009193e396df71cfcd16817eb0904

The job fails immediately:

[2026-03-27 01:47:34 UTC] Using base benchmarks ref: 641119486f8009193e396df71cfcd16817eb0904
[2026-03-27 01:47:35 UTC] ERROR: Failed to get base branch SHA (status 404): {"message":"Not Found",...}

Workaround

Create a branch from the SHA first, then pass the branch name instead.

Suggested Fix

In create_temporary_branch(), detect when BENCHMARKS_REF is a SHA (40-char hex string) and use it directly as the base_sha for branch creation, skipping the git/refs/heads/ lookup.

import re

if re.fullmatch(r'[0-9a-f]{40}', benchmarks_ref):
    base_sha = benchmarks_ref
else:
    # existing ref lookup logic
    ...

Start in orchestrate_eval.py at create_temporary_branch() and inspect the existing GitHub refs lookup. Reproduce the evaluation with the provided 40-character benchmarks_branch value, then verify that SHA-based inputs create the temporary branch without the 404 while branch-name inputs retain their existing behavior.