fix(hermes-agent): checkout scm in the amd64 build leg (fix exit 128)

The amd64 leg runs on its own fresh titan-24 pod but never checked out the SCM,
so its independent reviewed-revision boundary check hit 'git rev-parse
origin/main -> fatal: not a git repository' and the build failed with exit 128
(the arm64 leg built and pushed fine). Add checkout scm to the amd64 stage.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01BvMSXH8VH2tMWXanb8SJdf

ci/Jenkinsfile.hermes-agent-image

@@ -329,6 +329,10 @@ spec:
         }
       }
       steps {
+        // This amd64 leg runs on its own fresh pod (titan-24), so it must check
+        // out the SCM itself before the reviewed-revision git boundary check —
+        // otherwise `git rev-parse origin/main` fails with "not a git repository".
+        checkout scm
         container('jnlp') {
           sh '''
             set -eu