test(smoke): don't hard-code a 'v' prefix on release_tag

The smoke workflow deliberately passes RELEASE_TAG=smoke to the variant
build so smoke images don't collide with real vX.Y.Z tags. The variant
bakes that into /etc/pi-devbox/build-manifest.json, and pi-devbox-version
prints 'pi-devbox smoke' — correct behaviour. But the smoke assertion
required substring 'pi-devbox v', which only holds for real releases.

Assertion never fired before because pi-devbox-version was added after
v1.5.0 (fb49828, 2026-07-15) and every CI attempt since was blocked
before smoke ran (v1.6.0 network flake, v1.6.1 first successful base
build hit this). CI run 509 (v1.6.1) surfaced it.

Fix: require the substring 'pi-devbox ' (space, no v). The two
neighbouring assertions on --json and --quiet already cover the value
of release_tag; this one just verifies the human line renders.
Everything else in 55/56 checks passed on run 509 including
pi 0.81.1 reported and base-229f04e5d021 pushed OK.

scripts/smoke-test.sh

@@ -166,7 +166,7 @@ run "manifest has no unresolved ('unknown') components" \
 run "pi-devbox-version binary present + executable" \
     "test -x /usr/local/bin/pi-devbox-version"
 run_expect "pi-devbox-version human output shows release tag" \
-    "pi-devbox-version" "pi-devbox v"
+    "pi-devbox-version" "pi-devbox "
 run_expect "pi-devbox-version --json round-trips the manifest" \
     "pi-devbox-version --json" '"release_tag"'
 run_expect "pi-devbox-version --quiet is a compact one-liner" \