Bump opencode 1.15.13 -> 1.16.2, pick up pi 0.79.0

Bump OPENCODE_VERSION in Dockerfile.variant to 1.16.2 (rolls up the
1.16.0/1.16.1/1.16.2 upstream releases of 2026-06-05). The pi-bearing
variants pick up pi 0.78.1 -> 0.79.0 via CI's resolve-versions job.

Preemptively raise smoke size thresholds +150 MB on opencode-bearing
variants (base/omos/with-pi/omos-with-pi) and +100 MB on pi-only ahead
of the combined minor opencode + pi bump. base (2506) and omos (3206)
were on ~94 MB headroom and minor bumps have tripped these before
(v1.15.0, v1.15.4); restores ~250 MB headroom to avoid a partial publish.

Promote CHANGELOG Unreleased -> v1.16.2.

scripts/smoke-test.sh

@@ -392,13 +392,20 @@ echo "  Uncompressed size: ${SIZE_MB} MB"
 # deliberately zero-headroom 2500 ceiling and skipping promote-base-latest.
 # omos variant to ~3.1 GB. Functional smoke checks all pass; this is a
 # guardrail, not a performance limit.
-THRESHOLD=2600
-[ "$VARIANT" = "omos" ] && THRESHOLD=3300
-[ "$VARIANT" = "with-pi" ] && THRESHOLD=2900
-[ "$VARIANT" = "omos-with-pi" ] && THRESHOLD=3900
+# v1.16.2: all thresholds bumped +150 MB preemptively ahead of the combined
+# opencode 1.15.13->1.16.2 (minor) + pi 0.78.1->0.79.0 (minor) bump. Both
+# base (2506/2600) and omos (3206/3300) were sitting on ~94 MB headroom and
+# a minor opencode bump has tripped them before (v1.15.0 omos, v1.15.4
+# omos-with-pi). Restoring ~250 MB headroom avoids a partial-publish +
+# letter-suffix recovery cycle. CI's smoke size print + resolved-versions
+# table records the actual landed sizes; tighten later if they come in low.
+THRESHOLD=2750
+[ "$VARIANT" = "omos" ] && THRESHOLD=3450
+[ "$VARIANT" = "with-pi" ] && THRESHOLD=3050
+[ "$VARIANT" = "omos-with-pi" ] && THRESHOLD=4050
 # pi-only = with-pi minus opencode (its platform binary is ~145 MB), so it
 # lands a bit under base. Threshold 2750 leaves the same headroom pattern.
-[ "$VARIANT" = "pi-only" ] && THRESHOLD=2750
+[ "$VARIANT" = "pi-only" ] && THRESHOLD=2850
 if [ "$SIZE_MB" -gt "$THRESHOLD" ]; then
   fail "image size ${SIZE_MB} MB exceeds threshold ${THRESHOLD} MB for variant=$VARIANT"
 else