fix(ci): gate the artifact the provenance actually attests

The gate asked assayward to verify a digest that appeared in no attestation,
so it answered SUBJECT_DIGEST_MISMATCH and every downstream check then
reported its evidence missing. forgeseal attests the dependency SBOM as its
subject, so hash that file and gate on it, matching smithmark's working gate.

Author: sns45

.github/workflows/assayward-gate.yml

@@ -41,10 +41,18 @@ jobs:
             --attest \
             --keyed
 
-      - name: Compute artifact digest
+      # The gated artifact must be the one the provenance actually attests.
+      # `forgeseal pipeline --attest` records the dependency SBOM as its
+      # subject, so hashing the binary here produced a digest that appears in
+      # no attestation: assayward answered SUBJECT_DIGEST_MISMATCH and every
+      # downstream check then reported its evidence missing. Hashing the
+      # attested subject binds the policy to real evidence. (Never caught
+      # before because this workflow's trigger meant it never ran.)
+      - name: Compute attested subject digest
         id: digest
         run: |
-          DIGEST="sha256:$(sha256sum ./bin/forgeseal 2>/dev/null | awk '{print $1}' || echo 'unknown')"
+          test -f ./forgeseal-output/sbom.cdx.json
+          DIGEST="sha256:$(sha256sum ./forgeseal-output/sbom.cdx.json | awk '{print $1}')"
           echo "value=$DIGEST" >> "$GITHUB_OUTPUT"
 
       - name: Obtain svidmint SVID