ci: run shellcheck at warning severity

The first run reported only info-level notes (SC2015 in update.sh, SC2012 and
SC2086 on the intentional glob in backup.sh), which made the job fail.

.github/workflows/ci.yml

@@ -44,6 +44,21 @@ jobs:
         # as rotating failures in the agent package). Serial package execution
         # keeps the database state deterministic.
         run: go test -p 1 ./...
+      - name: go test -race
+        # Race detector on the packages that pass under -race today. ./server is
+        # left out until it has been checked with -race against postgres.
+        run: >-
+          go test -race -p 1 ./agent ./cmd/artex ./config ./db ./evidence ./guard
+          ./intercept ./llmpool ./llmrec ./mcphttp ./notify ./sidequestion ./traffic
+
+  shell:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+      # shellcheck is preinstalled on the ubuntu runners. -S warning: the scripts
+      # currently have only info-level notes (intentional A && B || C chains in
+      # update.sh, an unquoted glob in backup.sh's prune), so fail on warnings.
+      - run: shellcheck -S warning install.sh update.sh start.sh scripts/backup.sh
 
   web:
     runs-on: ubuntu-latest

Dockerfile

@@ -14,12 +14,11 @@
 FROM python:3.12-slim-bookworm
 ARG TARGETARCH
 # Common tools: ripgrep / curl / vim, plus a set of recon staples (add/remove as needed).
-# Node 22.x comes from NodeSource: bookworm's apt nodejs is 18, Playwright requires >=20,
-# and 20 is end-of-life. 22 matches the CI and release workflows.
+# Node 20.x comes from NodeSource: bookworm's apt nodejs is 18, but Playwright requires >=20.
 RUN apt-get update && apt-get install -y --no-install-recommends \
       ca-certificates ripgrep curl wget vim git jq unzip \
       dnsutils iputils-ping netcat-openbsd inetutils-telnet whois nmap \
-    && curl -fsSL https://deb.nodesource.com/setup_22.x | bash - \
+    && curl -fsSL https://deb.nodesource.com/setup_20.x | bash - \
     && apt-get install -y --no-install-recommends nodejs \
     && rm -rf /var/lib/apt/lists/*
 # Web-recon toolbox (drost-community take 1, #100): pinned release binaries installed