ci: Add ggshield GitHub Actions workflow and skip in pre-commit.ci

- Skip ggshield in pre-commit.ci (no secrets support)
- Add dedicated GitHub Actions workflow for ggshield scanning
- Requires GITGUARDIAN_API_KEY secret to be configured

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: tablackburn

.github/workflows/ggshield.yaml

@@ -0,0 +1,17 @@
+name: GitGuardian Scan
+
+on:
+  push:
+  pull_request:
+
+jobs:
+  scanning:
+    name: GitGuardian Scan
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+        with:
+          fetch-depth: 0
+      - uses: GitGuardian/ggshield-action@v1
+        env:
+          GITGUARDIAN_API_KEY: ${{ secrets.GITGUARDIAN_API_KEY }}

.pre-commit-config.yaml

@@ -1,3 +1,6 @@
+ci:
+  skip: [ggshield]
+
 repos:
   - repo: https://github.com/gitguardian/ggshield
     rev: v1.46.0