Add contribution guardrails

Add CI syntax check, CODEOWNERS, and contribution guidelines to keep the repo clean.

Author: william ghysels

.github/workflows/ci.yml

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+name: CI
+
+on:
+  push:
+  pull_request:
+
+jobs:
+  compile:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+      - uses: actions/setup-python@v5
+        with:
+          python-version: "3.11"
+      - name: Compile Python (syntax check)
+        run: |
+          python -m py_compile image_database.py visualize_umap.py

CODEOWNERS

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+* @droon

CONTRIBUTING.md

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+## Contributing
+
+Please open a pull request for any change.
+
+### Rules
+
+- Do not commit personal/local configuration (`config.json`) or any databases (`*.db`).
+- Keep all paths in docs/examples generic (`/path/to/...`).
+- Avoid committing generated files like `__pycache__/`.
+
+### Checks
+
+CI runs a lightweight syntax check (`python -m py_compile`) on `image_database.py` and `visualize_umap.py`.