build: extend the main-sync gate to the image build and the k8s deploy

Same hazard as the package build, with the cluster on the receiving end: a
branch behind main builds images missing whatever landed there, and deploy-k8s.sh
pins that sha in Pulumi — making the stale build the cluster's source of truth.

build-mcpd.sh gets its own call because it is run standalone as well as from
deploy-k8s.sh, so neither can rely on the other having checked.

`--dry-run` is exempt. It builds and cuts over nothing, and blocking a read-only
inspection is exactly what teaches people to export MCPCTL_ALLOW_BEHIND_MAIN=1
permanently — which would disable the gate for the real deploys too.

The failure text is now artifact-agnostic ("produce an artifact" / "shipping
it"), since one helper now speaks for packages, images and deploys.

Verified against a synthetic ref one commit ahead: build-mcpd.sh exits 1 before
any docker work, and deploy-k8s.sh exits 1 before the test gate, the pg_dump,
the image build and pulumi. Neither the working tree nor HEAD was moved to test
this — the ref was built with git commit-tree and deleted afterwards.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_019wUmrfkVQR6CKcYKxENq7k

scripts/build-mcpd.sh

@@ -16,6 +16,11 @@ if [ -f .env ]; then
   set -a; source .env; set +a
 fi
 
+# This pushes an image to the registry, so the same staleness gate as the package
+# builds applies. Run standalone as well as from deploy-k8s.sh, hence its own copy.
+source "$SCRIPT_DIR/check-main-sync.sh"
+check_main_sync
+
 # Push directly to internal address (external proxy has body size limit)
 REGISTRY="10.0.0.194:3012"
 IMAGE="mcpd"