Add CLAUDE.md with project conventions and structure

Author: nicolas2bert

CLAUDE.md

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+# CLAUDE.md
+
+## Project overview
+
+This is **scality/workflows**, a repository of reusable GitHub Actions workflows shared across the Scality organization. Downstream repos call these workflows via `workflow_call`.
+
+## Repository structure
+
+- `.github/workflows/` — Reusable workflow definitions (the core asset)
+- `docs/` — MkDocs Material documentation for each workflow
+- `tests/` — Dockerfiles used as fixtures to validate workflows on PR
+- `mkdocs.yml` — Documentation site configuration
+- `requirements.txt` — Python dependency for docs (`mkdocs-material`)
+
+## Workflows
+
+| File | Purpose |
+|------|---------|
+| `docker-build.yaml` | Build and push Docker images with Buildx, caching, multi-platform support |
+| `trivy.yaml` | Container vulnerability scanning, uploads SARIF to GitHub Security tab |
+| `lfs-warning.yaml` | Validates file sizes in PRs, warns about files not tracked by Git LFS |
+| `claude-code-review.yml` | AI-powered PR review via Vertex AI |
+
+## Conventions
+
+- Workflow files use `.yaml` extension (except `claude-code-review.yml`)
+- All workflows use `workflow_call` trigger with typed `inputs` and `secrets`
+- Secrets have sensible defaults where possible (e.g., `GITHUB_TOKEN` for registry auth)
+- Actions are pinned to major version tags (e.g., `@v6`, `@v3`)
+- `tests.yaml` calls all workflows locally (`./.github/workflows/...`) to validate on PR
+
+## Testing
+
+There is no test framework. Workflows are tested by `tests.yaml` which calls each reusable workflow with test fixtures from `tests/docker/`.
+
+## Documentation
+
+Documentation is built with MkDocs Material (`mkdocs build --strict`). When adding or modifying a workflow, update the corresponding page in `docs/`.
+
+## Downstream impact
+
+Changes to workflow `inputs`, `secrets`, or `outputs` can break consuming repos. Treat these as public API surfaces — avoid removing or renaming parameters without a migration path.