ci: generate production .env dynamically in CD workflow and update secrets documentation

.github/instructions/AGENTS.md

@@ -9,3 +9,13 @@ Agents instructions for the linearvoid project.
 - Update documentation first before implementing changes to ensure traceability of decisions.
 - Plan any task in the `TODO.md` file with clear steps and mark them as completed when done. Move completed sections to DONE.md. Write entry in root `CHANGELOG.md` for any significant changes. Avoid using in-memory todo lists or update them frequently to sync from the `TODO.md` file.
 - Before taking any action except for planning and writing `TODO.md`, always ask user for confirmation, accept `go`, `ok`, `yes`, `next`, or `continue` to proceed. If the user says `no`, `stop`, or `cancel`, abort the action and return to planning.
+
+## Security & Secrets Protection
+
+- **Never commit raw secrets or credentials** to the repository.
+- **Never print or log raw secrets** in terminal commands or output files.
+- **Use Gitea Actions Secrets** for all production credentials and environment variables.
+- **Avoid hardcoding sensitive values** in workflow files (`.yml`). Instead, reference them using `${{ secrets.SECRET_NAME }}`.
+- **Do not use the `GITEA_` prefix** for custom secrets or environment variables in Gitea, as it is reserved internally and will be silently ignored or returned as empty. Use the `CI_` prefix instead (e.g., `CI_REGISTRY_USER`, `CI_REGISTRY_TOKEN`).
+- **Keep `.env` gitignored** and use `.env.example` as the template for local development.
+- **For CD deployments**, generate the `.env` file dynamically on the runner using a secure step that populates values from Gitea Actions Secrets.