test+docs: assert the node major in smoke, and correct v1.8.13's agent-browser version

Two findings from a delegated read-only audit of this repo, both verified from the
filesystem before patching.

1. No test asserted the node major, so a node-24 bump would have passed the smoke
   suite SILENTLY. scripts/smoke-test.sh:94 was a bare `run "node" "node --version"`
   — exit-0 and non-empty output only, the printed version compared to nothing —
   while the line above it uses run_expect against $EXPECTED_PI_VERSION for pi. A
   reader skimming the suite would reasonably assume node regressions were covered.
   Worse, this is where the "node v22.23.2 verified" line in the v1.8.13 recreate
   notes came from: printed output, not an assertion.

   Now gated on EXPECTED_NODE_MAJOR, which CI derives from Dockerfile.base's ARG
   NODE_VERSION — the single source of truth (Dockerfile.base:557 is the ONLY hard
   pin in the repo; Dockerfile.variant has no node install at all). That also
   catches a stale cached layer whose node disagrees with the declared ARG.
   Unset => previous behaviour, so this is backward compatible.

   Verified two-sided rather than assumed: the sed derivation yields 22 (empty
   would have silently disabled the assertion, reintroducing the bug); grep -Fq
   "v22." matches v22.23.2; "v24." does NOT match, so a wrong major is caught; and
   "v2." does not prefix-collide. Workflow YAML re-parsed after editing (9 jobs).

2. The v1.8.13 entry claimed "the image's own 0.35.2" for agent-browser. The image
   ships 0.36.0: /usr/lib/node_modules/agent-browser/package.json says version
   0.36.0, engines.node >=24.0.0, and no 0.35.2 exists anywhere in the image. The
   claim was also internally incoherent, contrasting 0.36.0 against a version that
   is not present. Corrected in place with a visible note, since the entry is
   already released. The reasoning survives untouched: the engines floor really is
   vestigial, because /usr/bin/agent-browser is a prebuilt aarch64 ELF invoked
   directly and never through node — which is why 0.36.0 runs fine on 22.23.2.

scripts/smoke-test.sh

@@ -5,6 +5,7 @@
 #
 # Verifies:
 #   - pi binary present and (if EXPECTED_PI_VERSION set) matches CI's resolved version
+#   - node MAJOR matches Dockerfile.base's ARG NODE_VERSION (if EXPECTED_NODE_MAJOR set)
 #   - mempalace core matches the audited pin (if EXPECTED_MEMPALACE_VERSION set)
 #   - new v1.0.0 base additions (pandoc, graphviz, imagemagick, yq, tealdeer)
 #   - typst PDF engine for pandoc (v1.4.0) — `pandoc --pdf-engine=typst`
@@ -91,7 +92,18 @@ if [ -n "${EXPECTED_PI_VERSION:-}" ]; then
 else
   run "pi"              "pi --version"
 fi
-run "node"            "node --version"
+# Until 2026-09-07 this was a bare `run "node" "node --version"`, which asserts
+# only that the binary exists and exits 0 — the printed version was never
+# compared to anything. A node major bump would therefore have passed this suite
+# SILENTLY, while a reader skimming it would reasonably assume node regressions
+# were covered. EXPECTED_NODE_MAJOR closes that: CI derives it from
+# Dockerfile.base's ARG NODE_VERSION (the single source of truth), so this also
+# catches a stale cached layer whose node does not match the declared ARG.
+if [ -n "${EXPECTED_NODE_MAJOR:-}" ]; then
+  run_expect "node major matches Dockerfile ARG" "node --version" "v${EXPECTED_NODE_MAJOR}."
+else
+  run "node"            "node --version"
+fi
 run "git"             "git --version"
 run "aws"             "aws --version"
 run "uv"              "uv --version"