feat: add agents.md

AGENTS.md

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+# Agent Guidelines
+
+This file provides guidelines for agentic coding agents operating in this repository.
+
+## Quick Reference
+
+### Running Tests
+
+```bash
+# Run all tests (Go + shell scripts + linting)
+make
+
+# Run only Go tests
+make test-go
+
+# Run Go tests with verbose output
+go test -v ./...
+
+# Run a single Go test by name
+go test -v -run TestName ./...
+
+# Run only shell script tests (requires Docker)
+make unit-tests
+
+# Run specific linter
+make test-github      # GitHub workflow/action validation
+make test-json        # JSON validation
+make test-markdown    # Markdown linting
+make test-openapi     # OpenAPI spec validation
+make test-shellcheck  # Shell script linting
+```
+
+## Code Style Guidelines
+
+### General
+
+Read `.editorconfig` for formatting rules.
+
+### Go
+
+Use idiomatic go:
+
+- **Testing**: Uses `github.com/stretchr/testify/assert` for assertions
+- **Error handling**: Wrap errors with `errors.Join()` for multiple errors, use sentinel errors defined at package level
+- **JSON**: Use struct tags for serialization (e.g., `json:"euro"`)
+- **Formatting**: Run `go fmt` before committing; `go mod tidy` for dependencies
+
+### Shell Scripts
+
+- Use POSIX-compliant `sh` (not bash)
+- Always use `set -e` for error handling
+- Check required tools with `which` before use
+- Exit codes: 1 = file error, 2 = directory error, 3 = validation error
+- Quote variables: `"$VAR"` not `$VAR`
+
+Example pattern for validation:
+
+```sh
+set -e
+
+FILE=$1
+[ -f "$FILE" ] || { echo "Not a file: $FILE"; exit 1; }
+```
+
+## GitHub Workflows
+
+This repository relies on GitHub Actions for CI/CD and data updates.
+Workflows are located in `.github/workflows/`.
+
+### Essential Workflows
+
+| Workflow                      | Purpose                                                                                                                         |
+| ----------------------------- | ------------------------------------------------------------------------------------------------------------------------------- |
+| `merge-test.yaml`             | Runs `make` on all PRs to main. Auto-merges dependabot and automation bot PRs.                                                  |
+| `health-checks.yaml`          | Scheduled daily health checks. Calls `health-check-grid.yaml`, `health-check-networks.yaml`, and `health-check-integrity.yaml`. |
+| `health-check-grid.yaml`      | Validates grid configuration data                                                                                               |
+| `health-check-networks.yaml`  | Validates network configuration data                                                                                            |
+| `health-check-integrity.yaml` | Validates data integrity across datasets                                                                                        |
+| `pull-data.yaml`              | Fetches energy price data from Energi Data Service API                                                                          |
+| `pull-tariff-data.yaml`       | Fetches tariff data from Energinet                                                                                              |
+| `update-visualization.yaml`   | Updates static visualization files                                                                                              |
+| `update-api-constants.yaml`   | Updates API constant definitions                                                                                                |
+| `upgrade-go.yaml`             | Automated Go version upgrades                                                                                                   |
+
+### Workflow Patterns
+
+- Workflows use `on.schedule` for periodic tasks
+- Most workflows support `workflow_dispatch` for manual runs
+- Health check workflows publish results to `gh-pages` branch
+
+## Docker
+
+Docker and Docker Compose are required for running tests and development:
+
+```bash
+# Install dependencies and run all tests
+make deps
+make
+
+# Run a shell in the runner container
+make shell
+```
+
+The `compose.yml` defines services for:
+
+- `runner` - Main container for running scripts and tests
+- `builder` - For build operations
+- `swagger` - Local OpenAPI documentation
+
+## Maintenance
+
+When modifying this repository:
+
+1. **Adding new tests**: Add Go tests in `*_test.go` files. Add shell tests in `test/*.sh` and call them from the main Makefile target.
+
+2. **Adding new workflows**: Place in `.github/workflows/` and validate with `make test-github`.
+
+3. **Updating AGENTS.md**: If you add new commands, change code style, or add new workflows, update this file to reflect those changes. Run `make` to ensure all tests pass.
+
+4. **Dependencies**: Use `go mod tidy` for Go dependencies. Shell scripts should use tools available in the runner container (check `compose.yml` for the base image).
+
+## Project Structure
+
+```
+├── .github/
+│   ├── workflows/  # GitHub Actions workflows
+│   └── actions/    # Reusable GitHub Actions
+├── build/          # Generated build artifacts (gitignored)
+├── configuration/  # JSON configuration files
+├── resources/      # OpenAPI spec and static HTML
+├── src/            # Shell scripts for data operations
+├── test/           # Shell script tests
+├── test/fixtures/  # Shell test fixtures
+├── testdata/       # Go test fixtures (raw API responses)
+└── Makefile        # Build automation
+```