Add SECURITY, CONTRIBUTING, and CODE_OF_CONDUCT files

CODE_OF_CONDUCT.md

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+# Code of Conduct
+
+Everyone interacting in this repository's issues, pull requests, discussions, and related spaces is expected to follow the [dbt Community Code of Conduct](https://docs.getdbt.com/community/resources/code-of-conduct).

CONTRIBUTING.md

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+# Contributing
+
+This repository contains the materials for the **Hamburg dbt workshop** run by [dbt Labs](https://www.getdbt.com). It is intended for workshop participants, not for external contributions.
+
+## How changes are made
+
+Workshop participants push to their own branches during the workshop. The default branch is maintained by dbt Labs and is not open to external pull requests.
+
+If you notice an issue with the workshop materials, please [open an issue](https://github.com/dbt-labs/hamburg-dbt-workshop/issues) rather than submitting a pull request.
+
+## Code of Conduct
+
+Everyone interacting in this repository is expected to follow the [dbt Community Code of Conduct](https://docs.getdbt.com/community/resources/code-of-conduct).

SECURITY.md

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+# Security policy
+
+## Reporting a vulnerability
+
+Please report security issues **privately** so they can be triaged and fixed before public disclosure.
+
+- **dbt Labs responsible disclosure:** [dbt Labs Security Disclosure](https://www.getdbt.com/security/disclosure)
+- **GitHub:** If this repository is enabled for them, you may also use [GitHub Security advisories](https://github.com/dbt-labs/hamburg-dbt-workshop/security/advisories/new) for this repo.
+
+Do not open a public issue for an undisclosed security vulnerability.
+
+## Supported versions
+
+This repository is a workshop fixture and does not produce releases. Security fixes are applied to the default branch on a best-effort basis.