release: v1.6.2 — lift smoke size threshold (3500→3800)

Completes v1.6.1's studio publish. Run 512 shipped v1.6.1 non-studio
(3411 MB) cleanly, but smoke-studio failed the size gate at 3574 MB vs
the 3500 MB threshold. Threshold was set in v1.0.0 pre-agent-browser
(baseline was 3.20 GB local arm64 + 300 MB margin); v1.6.0 baked in
agent-browser + Chromium (+~291 MB net) but the threshold was never
lifted. v1.6.0 never got to smoke because of the network fault, so
nothing surfaced this until v1.6.1's smoke-studio.

Bump SIZE_THRESHOLD_MB to 3800 (~225 MB margin above observed studio
number, tight enough to still catch a genuine +GB regression). Refresh
the comment above the constant with the current baseline + run 512
actuals so future readers know where the number came from.

CI-only change; image bytes identical to v1.6.1 except for the manifest's
release_tag/source_revision. Not base-affecting.

scripts/smoke-test.sh

@@ -31,11 +31,14 @@ IMAGE="${1:?usage: $0 <image>}"
 PASS=0; FAIL=0
 # pi-devbox v1.0.0 (decoupled from opencode-devbox) added pandoc, graphviz,
 # imagemagick, yq, tealdeer, a baked /etc/tmux.conf, and the non-modal
-# editors nano + micro (~15 MB combined). Local arm64 build
-# observed 3.20 GB. CI amd64 builds may differ slightly; threshold below
-# carries +300 MB margin to absorb arch differences without false reds.
-# Tighten in a follow-up release once amd64 actuals are observed in CI logs.
-SIZE_THRESHOLD_MB=3500
+# editors nano + micro (~15 MB combined). v1.6.0 baked in agent-browser +
+# Playwright Chromium (~291 MB net after dropping the unused headless-shell
+# build), which lifted the baseline. CI amd64 actuals observed on run 512
+# (v1.6.1): 3411 MB non-studio, 3574 MB studio. Threshold below carries
+# ~225 MB margin above the studio number to absorb minor arch/build-cache
+# differences and small future growth without false reds, while still
+# catching an unexpected +GB regression.
+SIZE_THRESHOLD_MB=3800
 
 run() {
   local label="$1"; local cmd="$2"