ci: fail the gate when astro build logs an error

`astro build` exits 0 on a vite asset-resolution failure. That is how a
stale `@import` survived the cutover and stayed green through every gate
run: the build printed `[ERROR]`, returned 0, and the gate believed it.

Tee the build log and treat a logged error as a failure. Negative-tested
by reintroducing the import -- GATE 1, with both the vite error and the
new message.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

.agents/scripts/gate.sh

@@ -43,7 +43,20 @@ step "types"
 pnpm exec astro check
 
 step "build"
-pnpm run build
+# `astro build` exits 0 even when vite fails to resolve an asset: the cutover
+# left a stale `@import` in a moved stylesheet and every gate stayed green for
+# it. Treat a logged error as a failed build.
+build_log=$(mktemp)
+if ! pnpm run build 2>&1 | tee "$build_log"; then
+  rm -f "$build_log"
+  exit 1
+fi
+if grep -q '\[ERROR\]' "$build_log"; then
+  echo "gate: astro build logged an error and still exited 0. See above." >&2
+  rm -f "$build_log"
+  exit 1
+fi
+rm -f "$build_log"
 
 step "content contracts"
 pnpm run verify