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.

internal/exporters/html_template_data.go

@@ -0,0 +1,131 @@
+package exporters
+
+import (
+	"strings"
+
+	"golang.org/x/text/cases"
+	"golang.org/x/text/language"
+
+	"github.com/kjanat/articulate-parser/internal/models"
+	"github.com/kjanat/articulate-parser/internal/services"
+)
+
+// Item type constants.
+const (
+	itemTypeText           = "text"
+	itemTypeList           = "list"
+	itemTypeKnowledgeCheck = "knowledgecheck"
+	itemTypeMultimedia     = "multimedia"
+	itemTypeImage          = "image"
+	itemTypeInteractive    = "interactive"
+	itemTypeDivider        = "divider"
+)
+
+// templateData represents the data structure passed to the HTML template.
+type templateData struct {
+	Course   models.CourseInfo
+	ShareID  string
+	Sections []templateSection
+	CSS      string
+}
+
+// templateSection represents a course section or lesson.
+type templateSection struct {
+	Type        string
+	Title       string
+	Number      int
+	Description string
+	Items       []templateItem
+}
+
+// templateItem represents a course item with preprocessed data.
+type templateItem struct {
+	Type      string
+	TypeTitle string
+	Items     []templateSubItem
+}
+
+// templateSubItem represents a sub-item with preprocessed data.
+type templateSubItem struct {
+	Heading   string
+	Paragraph string
+	Title     string
+	Caption   string
+	CleanText string
+	Answers   []models.Answer
+	Feedback  string
+	Media     *models.Media
+}
+
+// prepareTemplateData converts a Course model into template-friendly data.
+func prepareTemplateData(course *models.Course, htmlCleaner *services.HTMLCleaner) *templateData {
+	data := &templateData{
+		Course:   course.Course,
+		ShareID:  course.ShareID,
+		Sections: make([]templateSection, 0, len(course.Course.Lessons)),
+		CSS:      defaultCSS,
+	}
+
+	lessonCounter := 0
+	for _, lesson := range course.Course.Lessons {
+		section := templateSection{
+			Type:        lesson.Type,
+			Title:       lesson.Title,
+			Description: lesson.Description,
+		}
+
+		if lesson.Type != "section" {
+			lessonCounter++
+			section.Number = lessonCounter
+			section.Items = prepareItems(lesson.Items, htmlCleaner)
+		}
+
+		data.Sections = append(data.Sections, section)
+	}
+
+	return data
+}
+
+// prepareItems converts model Items to template Items.
+func prepareItems(items []models.Item, htmlCleaner *services.HTMLCleaner) []templateItem {
+	result := make([]templateItem, 0, len(items))
+
+	for _, item := range items {
+		tItem := templateItem{
+			Type:  strings.ToLower(item.Type),
+			Items: make([]templateSubItem, 0, len(item.Items)),
+		}
+
+		// Set type title for unknown items
+		if tItem.Type != itemTypeText && tItem.Type != itemTypeList && tItem.Type != itemTypeKnowledgeCheck &&
+			tItem.Type != itemTypeMultimedia && tItem.Type != itemTypeImage && tItem.Type != itemTypeInteractive &&
+			tItem.Type != itemTypeDivider {
+			caser := cases.Title(language.English)
+			tItem.TypeTitle = caser.String(item.Type)
+		}
+
+		// Process sub-items
+		for _, subItem := range item.Items {
+			tSubItem := templateSubItem{
+				Heading:   subItem.Heading,
+				Paragraph: subItem.Paragraph,
+				Title:     subItem.Title,
+				Caption:   subItem.Caption,
+				Answers:   subItem.Answers,
+				Feedback:  subItem.Feedback,
+				Media:     subItem.Media,
+			}
+
+			// Clean HTML for list items
+			if tItem.Type == itemTypeList && subItem.Paragraph != "" {
+				tSubItem.CleanText = htmlCleaner.CleanHTML(subItem.Paragraph)
+			}
+
+			tItem.Items = append(tItem.Items, tSubItem)
+		}
+
+		result = append(result, tItem)
+	}
+
+	return result
+}