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.

admin/go.mod

@@ -3,10 +3,10 @@ module gitea.hostxtra.co.uk/mrhid6/vantage/admin
 go 1.26
 
 require (
+	gitea.hostxtra.co.uk/vantage/vantage-shared v0.1.0
 	github.com/gin-gonic/gin v1.10.0
 	github.com/google/uuid v1.6.0
 	github.com/joho/godotenv v1.5.1
-	gitea.hostxtra.co.uk/vantage/vantage-shared v0.0.0-00010101000000-000000000000
 	github.com/redis/go-redis/v9 v9.20.1
 	go.mongodb.org/mongo-driver/v2 v2.8.0
 	golang.org/x/crypto v0.54.0
@@ -51,5 +51,3 @@ require (
 	gopkg.in/check.v1 v1.0.0-20201130134442-10cb98267c6c // indirect
 	gopkg.in/yaml.v3 v3.0.1 // indirect
 )
-
-replace gitea.hostxtra.co.uk/vantage/vantage-shared => ../shared