ci(secret-scanner): make gitleaks gate for real, not continue-on-error

The gitleaks job carried continue-on-error: true (comment: gitleaks-action
tool-cache PATH injection unreliable on self-hosted runners), so the estate's
primary secret scanner could not fail a single PR anywhere it is used.

Replace gitleaks/gitleaks-action with a pinned, checksum-verified gitleaks
8.18.4 binary run directly (gitleaks detect --source . --no-git --exit-code 1).
A direct install is deterministic on every runner, removing the exact
unreliability the continue-on-error was papering over, so the scan now gates.

Comments/summary were already disabled via env vars, so the action provided
only the scan; the swap loses no functionality and drops the now-unneeded
GITHUB_TOKEN env and fetch-depth: 0.

Verified locally with the pinned binary: checksum matches; a private-key
fixture exits 1, a clean tree exits 0; actionlint clean.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Author: hyperpolymath

.github/workflows/secret-scanner-reusable.yml

@@ -85,20 +85,42 @@ jobs:
     # `contents: read`, which they get from the workflow-level block.
     permissions:
       contents: read
-      # gitleaks-action's `ScanPullRequest` posts a summary comment via
-      # the GitHub Issues/PR API. We disabled it via env vars below.
     steps:
       - uses: actions/checkout@9c091bb21b7c1c1d1991bb908d89e4e9dddfe3e0 # v7.0.0
-        with:
-          fetch-depth: 0
 
-      - name: Gitleaks Secret Scan
-        continue-on-error: true  # gitleaks-action tool-cache PATH injection unreliable on self-hosted runners
-        uses: gitleaks/gitleaks-action@e0c47f4f8be36e29cdc102c57e68cb5cbf0e8d1e # v3.0.0
-        env:
-          GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
-          GITLEAKS_ENABLE_COMMENTS: false
-          GITLEAKS_ENABLE_SUMMARY: false
+      # Gitleaks runs as a pinned, checksum-verified binary rather than via
+      # gitleaks/gitleaks-action. The action injects gitleaks into the runner
+      # tool-cache and prepends it to PATH, which is unreliable on self-hosted
+      # runners — that unreliability was the reason this step previously carried
+      # `continue-on-error: true`, i.e. the estate's primary secret scanner
+      # could not fail a PR. A pinned direct install is deterministic on every
+      # runner, so the scan gates for real.
+      - name: Install gitleaks (pinned + checksum-verified)
+        run: |
+          set -euo pipefail
+          GITLEAKS_VERSION=8.18.4
+          SHA256=ba6dbb656933921c775ee5a2d1c13a91046e7952e9d919f9bac4cec61d628e7d
+          cd "$RUNNER_TEMP"
+          curl -fsSL -o gitleaks.tar.gz \
+            "https://github.com/gitleaks/gitleaks/releases/download/v${GITLEAKS_VERSION}/gitleaks_${GITLEAKS_VERSION}_linux_x64.tar.gz"
+          echo "${SHA256}  gitleaks.tar.gz" | sha256sum -c -
+          tar -xzf gitleaks.tar.gz gitleaks
+          chmod +x gitleaks
+          ./gitleaks version
+
+      # Scans the checked-out working tree (--no-git): deterministic and fast.
+      # A finding exits non-zero and fails the check. Full-history scanning is
+      # available by removing --no-git and adding `fetch-depth: 0` to the
+      # checkout above, at the cost of a slower scan on every PR.
+      - name: Gitleaks secret scan (gating)
+        run: |
+          set -euo pipefail
+          "$RUNNER_TEMP/gitleaks" detect \
+            --source . \
+            --no-git \
+            --redact \
+            --no-banner \
+            --exit-code 1
 
   # Rust-specific: hardcoded crypto values. Self-skips when no Cargo.toml
   # is present, so this job is safe to run on non-Rust repos.