ci: push images to artifactapi registry instead of gitea

Hard switch of the docker push target from the Gitea registry to the
artifactapi local docker registry (docker-internal); the Gitea VM and its
registry are being retired. Drops the droneci/DRONECI_PASSWORD creds since
artifactapi accepts unauthenticated in-cluster pushes. Also repoints the agentpool sample image (covers forgebot-api + forgebot-operator).

Claude-Session: https://claude.ai/code/session_015ur3i7D2azsMAWTSVABApv

config/samples/agentpool.yaml

@@ -8,7 +8,7 @@ spec:
   model: claude-sonnet-4-20250514
   endpoint: https://litellm.k8s.syd1.au.unkin.net
   maxConcurrent: 3
-  image: git.unkin.net/unkin/agent-dev:latest
+  image: artifactapi.k8s.syd1.au.unkin.net/docker-internal/agent-dev:latest
   resources:
     requests:
       cpu: 500m