Add Gitleaks secret scanning

.github/workflows/gitleaks.yml

@@ -0,0 +1,57 @@
+name: Secret scan (Gitleaks)
+
+on:
+  push:
+  pull_request:
+
+permissions:
+  contents: read
+
+jobs:
+  gitleaks:
+    name: Scan changed commits for secrets
+    runs-on: ubuntu-latest
+    timeout-minutes: 10
+    steps:
+      - name: Check out full history
+        uses: actions/checkout@3d3c42e5aac5ba805825da76410c181273ba90b1 # v7.0.1
+        with:
+          fetch-depth: 0
+          persist-credentials: false
+
+      - name: Download and verify Gitleaks
+        shell: bash
+        run: |
+          set -euo pipefail
+          archive="$RUNNER_TEMP/gitleaks.tar.gz"
+          curl --fail --location --silent --show-error \
+            https://github.com/gitleaks/gitleaks/releases/download/v8.30.1/gitleaks_8.30.1_linux_x64.tar.gz \
+            --output "$archive"
+          printf '%s  %s\n' \
+            '551f6fc83ea457d62a0d98237cbad105af8d557003051f41f3e7ca7b3f2470eb' "$archive" \
+            | sha256sum --check --status
+          tar -xzf "$archive" -C "$RUNNER_TEMP" gitleaks
+          "$RUNNER_TEMP/gitleaks" version
+
+      - name: Scan new commits (redacted output)
+        shell: bash
+        env:
+          EVENT_NAME: ${{ github.event_name }}
+          EVENT_BEFORE: ${{ github.event.before }}
+          PR_BASE_SHA: ${{ github.event.pull_request.base.sha }}
+          COMMIT_SHA: ${{ github.sha }}
+        run: |
+          set -euo pipefail
+          base=''
+          if [[ "$EVENT_NAME" == "pull_request" ]]; then
+            base="$PR_BASE_SHA"
+          elif [[ -n "$EVENT_BEFORE" && ! "$EVENT_BEFORE" =~ ^0+$ ]]; then
+            base="$EVENT_BEFORE"
+          fi
+
+          if [[ -n "$base" ]] && git cat-file -e "${base}^{commit}" 2>/dev/null; then
+            "$RUNNER_TEMP/gitleaks" git --redact --log-opts="${base}..${COMMIT_SHA}" .
+          else
+            # Initial pushes have no before/base commit; scan available history.
+            "$RUNNER_TEMP/gitleaks" git --redact .
+          fi