feat: add .github/copilot-instructions.md

Co-authored-by: hairmare <116588+hairmare@users.noreply.github.com>

Author: Copilot

.github/copilot-instructions.md

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+# RaBe Backstage – Copilot Instructions
+
+This is the [Backstage](https://backstage.io/) instance for Radio Bern RaBe.
+It contains catalog, TechDocs, and RaBe SSO integration via Keycloak OIDC.
+
+## Repository Structure
+
+- `packages/app/` – React frontend (Backstage app)
+- `packages/backend/` – Node.js backend
+- `plugins/` – Custom Backstage plugins (e.g. `permission-backend-module-rabe`, `catalog-backend-module-transformer`)
+- `docs/` – MkDocs-based documentation
+- `app-config.yaml` – Main Backstage config
+- `app-config.production.yaml` – Production overrides
+
+## Development Flow
+
+Install dependencies:
+
+```sh
+yarn install
+```
+
+Start the app (frontend + backend):
+
+```sh
+yarn start
+```
+
+## Build, Test & Validate
+
+```sh
+# TypeScript type-check
+yarn tsc
+
+# Run unit tests
+yarn test
+
+# Run all unit tests with coverage
+yarn test:all
+
+# Lint changed files (since origin/main)
+yarn lint
+
+# Lint all files
+yarn lint:all
+
+# Build backend
+yarn build:backend
+
+# Build everything
+yarn build:all
+
+# Run Playwright e2e tests (requires Chromium)
+npx playwright install --with-deps chromium
+npx playwright test
+```
+
+## Code Standards
+
+- **Language**: TypeScript throughout; follow existing patterns in each package
+- **License**: GNU AGPL-3.0-only — include the SPDX identifier in new files where applicable
+- **Formatting**: Prettier (config from `@backstage/cli/config/prettier`); run `prettier --write .` or let `lint-staged` handle it
+- **Linting**: ESLint with Backstage's default config (`.eslintrc.js`)
+- **Commits**: Follow [Conventional Commits](https://www.conventionalcommits.org/en/v1.0.0/)
+  - `fix:` → PATCH release
+  - `feat:` → MINOR release
+  - `BREAKING CHANGE:` footer → MAJOR release
+  - Other types (e.g. `chore:`, `docs:`, `refactor:`) do not trigger a release
+  - Releases are automated via [go-semantic-release](https://go-semantic-release.xyz/) (see <https://go-semantic-release.xyz/llms.txt>)
+
+## Key Guidelines
+
+1. Keep changes minimal and surgical — avoid unrelated modifications
+2. Write unit tests for new functionality; place them alongside the source file as `*.test.ts` / `*.test.tsx`
+3. Playwright e2e tests live in `packages/app/e2e-tests/`
+4. Use `yarn backstage-cli versions:bump` to update Backstage dependencies
+5. New plugins go in `plugins/` and must be added to the relevant workspace package
+6. Container image is published from `Dockerfile` via CI; do not break the Docker build