Add repository-specific Copilot agent instructions

.github/copilot-instructions.md

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+# Copilot instructions for `precimed/container_template`
+
+## Repository purpose
+- This repository is a reusable template for containerized projects (Docker/Singularity/Apptainer) and workflow scaffolding.
+- Keep changes minimal and scoped to the issue; avoid broad template rewrites unless explicitly requested.
+
+## Key project structure
+- `containers/`: container recipes and built image references.
+- `tests/`: pytest-based validation for runtime/container behavior.
+- `scripts/init.py`: initialization script that customizes this template for downstream projects.
+- `.github/workflows/`: CI checks (including Python linting and container workflows).
+
+## Validation commands
+- Install test/lint dependencies:
+  - `python -m pip install -r test-requirements.txt flake8`
+- Run tests:
+  - `python -m pytest -q`
+- Run lint:
+  - `flake8 .`
+
+## Contribution expectations for agent changes
+- Prefer surgical edits over refactors.
+- Do not modify unrelated files to fix pre-existing lint issues unless the task asks for it.
+- If modifying behavior, update or add focused tests under `tests/`.
+- Keep workflow and container changes backward-compatible with template usage.