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.

vantagectl/Dockerfile

@@ -1,21 +1,26 @@
 # Build stage
 #
-# Context is the repository root, not vantagectl/, because vantagectl depends on
-# the shared module through a replace directive.
+# Context is vantagectl/ itself. It used to be the repository root, so that
+# shared/ could be copied in beside it; shared is now the private module
+# gitea.hostxtra.co.uk/vantage/vantage-shared, fetched like any other
+# dependency. The credential for it arrives as a BuildKit secret rather than a
+# build arg, which would be baked into this stage's layer history.
 FROM golang:1.26 AS builder
 
 WORKDIR /src
 
-# Manifests first so the dependency layer caches independently of source edits.
-COPY shared/go.mod shared/go.sum ./shared/
-COPY vantagectl/go.mod vantagectl/go.sum ./vantagectl/
-RUN cd vantagectl && go mod download
+ENV GOPRIVATE=gitea.hostxtra.co.uk/*
 
-COPY shared/ ./shared/
-COPY vantagectl/ ./vantagectl/
+# Manifests first so the dependency layer caches independently of source edits.
+COPY go.mod go.sum ./
+RUN --mount=type=secret,id=netrc,target=/root/.netrc \
+    go mod download
+
+COPY . .
 
 ARG VERSION=dev
-RUN cd vantagectl && CGO_ENABLED=0 GOOS=linux go build \
+RUN --mount=type=secret,id=netrc,target=/root/.netrc \
+    CGO_ENABLED=0 GOOS=linux go build \
       -ldflags="-s -w -X main.Version=${VERSION}" -o /vantagectl .
 
 # Staged so the scratch image below can have a /tmp. It cannot mkdir one