Add SECURITY.md and CI lint workflow

.github/workflows/lint.yml

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+name: Lint
+
+on:
+  pull_request:
+    branches: [main]
+  push:
+    branches: [main]
+
+jobs:
+  lint:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+
+      - uses: actions/setup-python@v5
+        with:
+          python-version: "3.11"
+
+      - name: Install ruff
+        run: pip install ruff
+
+      - name: Run ruff
+        run: ruff check .

SECURITY.md

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+# Security Policy
+
+## Supported Versions
+
+| Version | Supported |
+|---------|-----------|
+| 0.1.x   | ✅        |
+
+## Reporting a Vulnerability
+
+Please **do not** open a public GitHub issue for security vulnerabilities.
+
+Use one of these channels instead:
+
+- **GitHub private vulnerability reporting** (preferred): go to the
+  [Security tab](https://github.com/Botrops1/FusionConstraints/security/advisories/new)
+  and open a private advisory.
+- **Email**: contact the maintainer directly (see profile).
+
+### What to include
+
+- Description of the vulnerability and potential impact
+- Steps to reproduce or proof-of-concept
+- Affected version(s)
+- Any suggested fix if you have one
+
+### Response timeline
+
+- Acknowledgement within **48 hours**
+- Assessment and fix timeline communicated within **7 days**
+
+Thank you for helping keep FusionConstraints users safe.