Initial reusable security-scan workflow

Semgrep SAST + Trivy filesystem scan, runs in parallel.
Either job failing fails the calling workflow.

Inputs:
- severity (Trivy threshold, default CRITICAL,HIGH)
- semgrep-config (rule pack, default auto)

.gitea/workflows/security-scan.yml

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+name: Security Scan (reusable)
+
+# Reusable workflow consumed by per-repo security.yml stubs across the
+# Hellion stack. Runs Semgrep SAST and Trivy filesystem scan in parallel.
+# Either job failing fails the calling workflow.
+
+on:
+  workflow_call:
+    inputs:
+      severity:
+        description: 'Trivy severity threshold (e.g. CRITICAL,HIGH or just CRITICAL)'
+        required: false
+        type: string
+        default: 'CRITICAL,HIGH'
+      semgrep-config:
+        description: 'Semgrep config (default auto detects rules per language)'
+        required: false
+        type: string
+        default: 'auto'
+
+jobs:
+  semgrep:
+    name: Semgrep SAST
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v4
+
+      - name: Set up Python
+        uses: actions/setup-python@v5
+        with:
+          python-version: '3.x'
+
+      - name: Install Semgrep
+        run: pip install --no-cache-dir semgrep
+
+      - name: Run Semgrep scan
+        # --config=auto pulls language-appropriate rule packs from semgrep.dev
+        # without requiring an account. --error makes the job fail when
+        # findings at or above the chosen severity exist.
+        run: semgrep scan --config=${{ inputs.semgrep-config }} --error --severity=ERROR --severity=WARNING
+
+  trivy:
+    name: Trivy Vulnerability Scan
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v4
+
+      - name: Run Trivy filesystem scan
+        # Scans dependency manifests (NuGet, npm, package-lock etc.) against
+        # the NVD CVE database. ignore-unfixed skips findings that have no
+        # patched version available so we focus on actionable items.
+        uses: aquasecurity/trivy-action@master
+        with:
+          scan-type: fs
+          scan-ref: .
+          severity: ${{ inputs.severity }}
+          exit-code: '1'
+          ignore-unfixed: true

README.md

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+# security-workflows
+
+Reusable Gitea Actions workflows for security scanning across Hellion repos. One central definition that all consumer repos call via a tiny stub.
+
+## Usage
+
+In any consumer repo, add `.gitea/workflows/security.yml`:
+
+```yaml
+name: Security
+on:
+  push:
+    branches: [main, master]
+  pull_request:
+  schedule:
+    - cron: '0 6 * * 1'   # weekly Monday 06:00 UTC
+  workflow_dispatch:
+
+jobs:
+  scan:
+    uses: JonKazama-Hellion/security-workflows/.gitea/workflows/security-scan.yml@main
+```
+
+That is the entire stub. Five lines of `jobs:` plus the trigger config the consumer actually wants.
+
+## What runs
+
+| Tool | What it does |
+|---|---|
+| **Semgrep** (auto-config) | SAST scanning for common vulnerability patterns across C#, JavaScript, TypeScript, Python, Go and more. Pulls language-appropriate rule packs from semgrep.dev. |
+| **Trivy** (filesystem scan) | Dependency vulnerability scanning against the NVD CVE database. Picks up NuGet `*.csproj`, npm `package.json`/`package-lock.json`, Dockerfiles and more. |
+
+Both jobs run in parallel. Either failing fails the calling workflow.
+
+## Tuning per consumer
+
+The reusable workflow accepts two optional inputs:
+
+```yaml
+jobs:
+  scan:
+    uses: JonKazama-Hellion/security-workflows/.gitea/workflows/security-scan.yml@main
+    with:
+      severity: 'CRITICAL'         # default 'CRITICAL,HIGH'
+      semgrep-config: 'p/owasp-top-ten'   # default 'auto'
+```
+
+`severity` is the Trivy threshold, `semgrep-config` swaps the rule pack (e.g. `p/owasp-top-ten`, `p/javascript`, `p/csharp`).
+
+## Pinning
+
+Consumers reference `@main` for rolling updates. To pin a specific commit, use the SHA: `@<sha>`. There are no tagged releases yet, the workflow API is intentionally minimal so breaking changes should be rare.