refactor: consume vantage-shared as an external private module

shared/ is extracted to gitea.hostxtra.co.uk/vantage/vantage-shared and
pinned at v0.1.0 by server, agent, admin, sitesvc and vantagectl. The
replace directives and the ./shared entry in go.work are gone.

Every Go build now needs a credential for the private module: CI writes a
netrc per job from REGISTRY_USER + RELEASE_TOKEN and sets GOPRIVATE, and
the four Go Dockerfiles take it as a BuildKit secret rather than a build
arg, which would survive in the builder layer's history. RELEASE_TOKEN
needs read access to the vantage org.

admin, sitesvc and vantagectl now build from their own directory; only
server still needs the repository root, for default_steps/. The rebuild
triggers in server-deploy.yml lose their shared/ patterns, since a
service now moves when its own go.mod pin does.

server/go.sum

@@ -1,3 +1,5 @@
+gitea.hostxtra.co.uk/vantage/vantage-shared v0.1.0 h1:H6PCb8JHucrRiqPe9kGOhXUjBD66tKFHCP3qz5TjdZc=
+gitea.hostxtra.co.uk/vantage/vantage-shared v0.1.0/go.mod h1:dWjeOFLltQ8sv9Pnn1xRxGfWGgqa2fkG0esuaJLoPXQ=
 github.com/aquasecurity/bolt-fixtures v0.0.0-20200903104109-d34e7f983986 h1:2a30xLN2sUZcMXl50hg+PJCIDdJgIvIbVcKqLJ/ZrtM=
 github.com/aquasecurity/bolt-fixtures v0.0.0-20200903104109-d34e7f983986/go.mod h1:NT+jyeCzXk6vXR5MTkdn4z64TgGfE5HMLC8qfj5unl8=
 github.com/aquasecurity/trivy-db v0.0.0-20260713131703-4be526083c54 h1:4CZNoDkNfcuACevZeDraACGmP1+L0nKkRY52+jV8k1M=