ci: security scanning, with the findings read and justified

.github/gitleaks.toml

@@ -0,0 +1,12 @@
+# gitleaks (keel-linux/.github, security-scan.yml): the default rules, less
+# the value below: KEEL_APT_KEY in the Makefile is the OpenPGP fingerprint of
+# the archive staging signing subkey, public by design (it names the key the
+# keyring must hold), not a secret.
+[extend]
+useDefault = true
+
+[[allowlists]]
+description = "public OpenPGP fingerprint of the Keel archive staging key"
+regexes = [
+  '''8CFD1A4841448B2227341CEB202CACBD0E97090A''',
+]

.github/workflows/security.yml

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+# Security scanning (keel-linux/.github, profile/WORKFLOWS.md "Security
+# scanning"): gitleaks, bandit, semgrep and shellcheck on GitHub-hosted
+# runners, no secrets. Findings that are not a risk are listed, each with its
+# justification, in .github/security-baseline; gitleaks fixtures, if any, in
+# .github/gitleaks.toml.
+name: security
+on:
+  pull_request:
+  push:
+    branches: [main]
+  workflow_dispatch:
+permissions:
+  contents: read
+jobs:
+  security:
+    uses: keel-linux/.github/.github/workflows/security-scan.yml@main
+    permissions:
+      contents: read
+      security-events: write