smoke-test: docker exec -u developer for runtime deployment checks

Without -u, docker exec runs as root and $HOME expands to /root, so
the test looks for ~/.pi/agent/keybindings.json under /root instead
of /home/developer. install.sh actually deploys correctly — the test
was just probing the wrong home directory.

Match opencode-devbox/scripts/smoke-test.sh's pattern.

scripts/smoke-test.sh

@@ -68,7 +68,7 @@ done
 
 exec_test() {
   local label="$1"; local cmd="$2"
-  if docker exec "$CID" sh -c "$cmd" >/dev/null 2>&1; then
+  if docker exec -u developer "$CID" sh -c "$cmd" >/dev/null 2>&1; then
     printf "  ✅  %s\n" "$label"; PASS=$((PASS+1))
   else
     printf "  ❌  %s\n" "$label"; FAIL=$((FAIL+1))