fix(lint): resolve golangci-lint failures and modernize string handling

CI's golangci-lint (v2.12.2) job was failing with 14 issues, which blocked
the dependent test job. This addresses all of them:

- goconst: extract repeated string literals into constants
  - format aliases ("md", "word", "htm") in the exporter factory
  - "section" lesson type shared by markdown and HTML exporters
  - default Articulate Rise base URL and host in the parser
  - reuse existing itemType* constants in the markdown switch
- staticcheck (QF1012): replace buf.WriteString(fmt.Sprintf(...)) with
  fmt.Fprintf(...) in the markdown exporter

Also drop the hardcoded `go: "1.24"` from .golangci.yml so the target Go
version is autodetected from go.mod.

internal/exporters/html_template_data.go

@@ -21,6 +21,9 @@ const (
 	itemTypeDivider        = "divider"
 )
 
+// lessonTypeSection identifies a lesson that acts as a section header.
+const lessonTypeSection = "section"
+
 // templateData represents the data structure passed to the HTML template.
 type templateData struct {
 	Course   models.CourseInfo
@@ -74,7 +77,7 @@ func prepareTemplateData(course *models.Course, htmlCleaner *services.HTMLCleane
 			Description: lesson.Description,
 		}
 
-		if lesson.Type != "section" {
+		if lesson.Type != lessonTypeSection {
 			lessonCounter++
 			section.Number = lessonCounter
 			section.Items = prepareItems(lesson.Items, htmlCleaner)