smoke-test: query /usr prefix for npm ls -g

The npm-prefix-on-volume fix (commit 9df126c) sets
NPM_CONFIG_PREFIX=/home/developer/.pi/npm-global in the image ENV so
user-installed pi packages survive container recreation. Side effect:
default 'npm ls -g' now queries the user prefix, missing the baked
opencode/pi/omos binaries that live in /usr.

The smoke test's oh-my-opencode-slim check ran 'npm ls -g | grep ...'
and started failing on validate-omos / validate-omos-with-pi after the
prefix fix landed on main, even though the package itself is correctly
installed and runnable.

Fix: explicitly invert the prefix per-call:
  NPM_CONFIG_PREFIX=/usr npm ls -g --depth=0 | grep ...

Other smoke checks (opencode --version, pi --version, bun --version)
go through PATH which already includes /usr/bin, so they were
unaffected. Only oh-my-opencode-slim was checked via npm rather than
a binary-on-PATH because it's a library, not a CLI.

scripts/smoke-test.sh

@@ -185,8 +185,13 @@ if [ "$VARIANT" = "omos" ] || [ "$VARIANT" = "omos-with-pi" ]; then
   run "bun (omos)"            "bun --version"
   run "bunx symlink (omos)"   "test -L /usr/local/bin/bunx && readlink /usr/local/bin/bunx"
   # oh-my-opencode-slim is npm-installed globally (not a bun install);
-  # verify it shows up in the global module list.
-  run "oh-my-opencode-slim"   "npm ls -g --depth=0 2>/dev/null | grep oh-my-opencode-slim"
+  # verify it shows up in the global module list. We must explicitly point
+  # npm at the system prefix (/usr) here: the image's NPM_CONFIG_PREFIX env
+  # is set to /home/developer/.pi/npm-global so user-installed packages
+  # land on the persistent volume — which means a default `npm ls -g`
+  # queries the user prefix and would miss the baked binaries even though
+  # they're correctly on PATH at /usr/bin.
+  run "oh-my-opencode-slim"   "NPM_CONFIG_PREFIX=/usr npm ls -g --depth=0 2>/dev/null | grep oh-my-opencode-slim"
 else
   if docker run --rm --entrypoint="" "$IMAGE" sh -c "command -v bun" >/dev/null 2>&1; then
     fail "bun should NOT be in base image but was found"