feat: add Semgrep SAST workflow for OpenSSF compliance

Adds Semgrep static analysis to satisfy SAST scorecard check.
CodeQL doesn't support Scheme, Semgrep provides broader coverage.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

Author: Jonathan D.A. Jewell

.github/workflows/semgrep.yml

@@ -0,0 +1,35 @@
+# SPDX-License-Identifier: AGPL-3.0-or-later
+name: Semgrep SAST
+
+on:
+  push:
+    branches: [main, master]
+  pull_request:
+    branches: [main, master]
+  schedule:
+    - cron: '0 5 * * 1'
+  workflow_dispatch:
+
+permissions: read-all
+
+jobs:
+  semgrep:
+    runs-on: ubuntu-latest
+    permissions:
+      security-events: write
+      contents: read
+    container:
+      image: semgrep/semgrep
+    steps:
+      - uses: actions/checkout@11bd71901bbe5b1630ceea73d27597364c9af683 # v4.2.2
+
+      - name: Run Semgrep
+        run: semgrep scan --sarif --output=semgrep.sarif --config=auto .
+        env:
+          SEMGREP_APP_TOKEN: ${{ secrets.SEMGREP_APP_TOKEN }}
+
+      - name: Upload SARIF
+        uses: github/codeql-action/upload-sarif@48ab28a6f5dbc2a99bf1e0131198dd8f1df78169 # v3.28.0
+        with:
+          sarif_file: semgrep.sarif
+        if: always()