ci(security): scan new commits on push/PR and the current tree weekly

`base: main` made every push to main rescan the whole history. Nova's own
Infura keys, removed from the files in #72 but still in history and live,
kept Secret Scan red on every push since at least 2026-09-21, so a new leak
would have read as the same old red.

- push and pull_request: no `base`, so the action scans only the commits the
  event brings (before..after, or the PR's base..head)
- schedule: scan a `git archive HEAD` copy of today's tree
- pin the action to v3.99.2 by commit and the image to 3.99.2 (was @main and
  latest)

.github/workflows/security.yml

@@ -22,13 +22,28 @@ jobs:
         with:
           fetch-depth: 0
 
-      - name: TruffleHog Secret Scan
-        uses: trufflesecurity/trufflehog@main
+      # Push and pull_request scan only the commits they bring: the action picks
+      # the range from the event. `base: main` made a push to main rescan the
+      # whole history, where Nova's own Infura keys (removed from the files in
+      # #72, still live and public upstream) kept this job red on every push
+      # since September, so a new leak would have looked like the old red.
+      - name: TruffleHog Secret Scan (new commits)
+        if: github.event_name != 'schedule'
+        uses: trufflesecurity/trufflehog@e76dff8a7e57d6df0c60ad8cbf5d5cba505a8fc9 # v3.99.2
         with:
           path: ./
-          base: ${{ github.event.repository.default_branch }}
+          version: 3.99.2
           extra_args: --only-verified
 
+      # The weekly run checks what is in the tree today, not the history.
+      - name: TruffleHog Secret Scan (current tree)
+        if: github.event_name == 'schedule'
+        run: |
+          mkdir -p "$RUNNER_TEMP/tree"
+          git archive HEAD | tar -x -C "$RUNNER_TEMP/tree"
+          docker run --rm -v "$RUNNER_TEMP/tree:/tree" ghcr.io/trufflesecurity/trufflehog:3.99.2 \
+            filesystem /tree --fail --no-update --github-actions --only-verified
+
   hardcoded-secrets:
     name: Hardcoded Secret Detection
     runs-on: ubuntu-latest