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.mod

@@ -37,7 +37,7 @@ require (
 )
 
 require (
-	gitea.hostxtra.co.uk/vantage/vantage-shared v0.0.0
+	gitea.hostxtra.co.uk/vantage/vantage-shared v0.1.0
 	github.com/bytedance/sonic v1.11.6 // indirect
 	github.com/bytedance/sonic/loader v0.1.1 // indirect
 	github.com/cespare/xxhash/v2 v2.3.0 // indirect
@@ -77,5 +77,3 @@ require (
 	google.golang.org/protobuf v1.34.2 // indirect
 	gopkg.in/yaml.v3 v3.0.1 // indirect
 )
-
-replace gitea.hostxtra.co.uk/vantage/vantage-shared => ../shared