chore: replace em dashes with hyphens, add no-em-dash rule to CLAUDE.md

server/Dockerfile

@@ -25,7 +25,7 @@ RUN --mount=type=secret,id=netrc,target=/root/.netrc \
       -ldflags="-s -w -X main.Version=${VERSION}" -o /vantage-server ./cmd
 
 # Staged so the scratch image below can have a /tmp. It cannot mkdir one
-# itself — scratch has no shell — and os.MkdirTemp fails outright without it.
+# itself - scratch has no shell - and os.MkdirTemp fails outright without it.
 RUN mkdir -p /staging/tmp && chmod 1777 /staging/tmp
 
 # Runtime stage