refactor(exporter): rewrite HTML exporter to use Go templates

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.

.golangci.yml

@@ -188,6 +188,12 @@ linters:
           - gochecknoglobals
           - gochecknoinits
 
+      # Exclude var-naming for interfaces package (standard Go pattern for interface definitions)
+      - path: internal/interfaces/
+        text: "var-naming: avoid meaningless package names"
+        linters:
+          - revive
+
       # Allow fmt.Print* in main package
       - path: ^main\.go$
         text: "use of fmt.Print"