chore: add CONTRIBUTING.md

CONTRIBUTING.md

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+# Contributing to Constellation
+
+Thanks for your interest in the Constellation identity protocol. This document covers local development, testing, and PR workflow.
+
+## Development setup
+
+```sh
+git clone https://github.com/cogos-dev/constellation.git
+cd constellation
+go mod download
+go build ./...
+```
+
+Requirements: Go 1.24+.
+
+## Running tests
+
+```sh
+go vet ./...
+golangci-lint run
+go test ./... -race
+```
+
+CI runs lint + race-enabled tests on every PR (`.github/workflows/ci.yml`).
+
+## Project layout
+
+- `cmd/constellation/` — CLI entry point
+- `internal/` — protocol primitives: ledger, signatures, peer state
+- `pkg/` — public API surface (import stable)
+- `testdata/` — fixtures for scenario tests
+
+## Submitting changes
+
+1. Fork the repo and create a branch from `main`
+2. Make your changes
+3. Run lint + tests above
+4. Update `CHANGELOG.md` under the Unreleased section if user-visible
+5. Open a pull request using the org PR template
+
+Commit messages: conventional-commits (`feat:`, `fix:`, `chore:`, etc.) preferred.
+
+## Reporting issues
+
+Use the org-level [Bug Report](https://github.com/cogos-dev/constellation/issues/new?template=bug.yml) or [Feature Request](https://github.com/cogos-dev/constellation/issues/new?template=feature.yml) forms.
+
+## License
+
+By contributing, you agree that your contributions will be licensed under the [MIT License](LICENSE).