feat(security): enforce declared cosign image signatures at the pull sites

New container::image_verify gates PodmanClient::pull_image and the
dev-only DockerRuntime::pull_image. Signature claims classify three
ways: absent/empty (pull unverified, logged), the literal
'cosign://...' placeholder every fleet manifest carries today (same —
enforcement stays dormant until the signing ceremony ships real
values), or a declared signature, which must verify via
'cosign verify --key /etc/archipelago/cosign.pub
--insecure-ignore-tlog=true' (plus --allow-insecure-registry
--allow-http-registry for the HTTP mirror; flags checked against
cosign's own docs) before anything is fetched. Missing key, missing
cosign binary, timeout, or verification failure all hard-fail the
pull — a declared signature cannot be skipped on either runtime. Key
path overridable via ARCHIPELAGO_COSIGN_PUBKEY for tests/staging.

Deletes security::ImageVerifier: zero callers, blocking
std::process::Command on would-be async paths, and a fantasy
'cosign verify --signature' invocation (that flag belongs to
verify-blob).

Activation ships with the Workstream B ceremony, in order: pin
cosign.pub on nodes + install cosign, then publish real
image_signature values in the catalog.

Tests: archipelago-container 58/58 (5 new), archipelago container::
159/159, security check clean.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

core/container/src/runtime.rs

@@ -546,7 +546,12 @@ impl DockerRuntime {
 
 #[async_trait]
 impl ContainerRuntime for DockerRuntime {
-    async fn pull_image(&self, image: &str, _signature: Option<&str>) -> Result<()> {
+    async fn pull_image(&self, image: &str, signature: Option<&str>) -> Result<()> {
+        // Same signature gate as the podman path — the docker fallback is
+        // dev-only, but a declared signature must never be skippable by
+        // switching runtimes.
+        crate::image_verify::enforce_signature_claim(image, signature, false).await?;
+
         let mut cmd = self.docker_async();
         cmd.arg("pull").arg(image);