Replaces the manual string-building implementation of the HTML exporter with a more robust and maintainable solution using Go's `html/template` package. This improves readability, security, and separation of concerns.
- HTML structure and CSS styles are moved into their own files and embedded into the binary using `go:embed`.
- A new data preparation layer adapts the course model for the template, simplifying rendering logic.
- Tests are updated to reflect the new implementation, removing obsolete test cases for the old string-building methods.
Additionally, this commit:
- Adds an `AGENTS.md` file with development and contribution guidelines.
- Updates `.golangci.yml` to allow standard Go patterns for interface package naming.
