refact(frank-dragoon): serve gates run on the shared gate library

The hand-written image/ck/tokenizer gates are require_image, require_bytes and
require_dir_files from scripts/lib/llm-gates.sh. The serve seam is unchanged
and still green (its podman stub now models `podman image exists`, the
primitive the gate uses). Behaviour preserved: every refusal names the
artifact, the endpoint and the fetch script.

tests/frank_dragoon_unit.bats

@@ -45,6 +45,13 @@ podman_stub() {
 #!/usr/bin/env bash
 case "$1" in
     images) printf '%s\n' "${STUB_DIGESTS:-}" ;;
+    image)
+        # `podman image exists REF` — exit 1 when storage lacks the ref,
+        # which is exactly the primitive the shared gate library calls.
+        case " ${STUB_DIGESTS:-} " in
+            *" $3 "*) exit 0 ;;
+            *)        exit 1 ;;
+        esac ;;
     rm)     exit 0 ;;
     run|wait) echo "PODMAN $*"; exit "${STUB_WAIT_RC:-0}" ;;
     *)      exit 0 ;;