audit: re-pin previous_audited_sha to a commit upstream still has

vllm-omni rewrote its main after 2026-09-05: the audited baseline's
`previous_audited_sha` (1015b1ced, "rebase: align vllm-omni with vLLM
d2906cc19586") is no longer reachable from any upstream ref, so every
pull_request run of the release drift audit dies at `git rev-parse` with
"Needed a single revision" before auditing anything (first seen on #140).

Pin `previous_audited_sha` to 66bbebd9d, the merge base of the lost commit
and current upstream main. `audited_sha` (44d3ae1) is unchanged and still on
main. Verified locally against a fresh upstream clone:

    audit 66bbebd9 -> 44d3ae10, enforce: issues 0, RESULT: CLEAN

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_018MAFmwP3u5qAMDgHfM6LFi

Author: tzhouam

adapters/vllm_omni/release_baseline.yaml

@@ -2,7 +2,7 @@ schema_version: 1
 
 upstream:
   repository: vllm-project/vllm-omni
-  previous_audited_sha: 1015b1ced073195ed19f0610264dfdf8a8150e6c
+  previous_audited_sha: 66bbebd9d3ef669b93fd7e39587672b20efc5933
   audited_ref: main
   audited_sha: 44d3ae100afa8411770b8b4d9442318bc5e897b3