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/services/parser.go

@@ -15,6 +15,14 @@ import (
 	"github.com/kjanat/articulate-parser/internal/models"
 )
 
+// Default endpoint configuration for the Articulate Rise API.
+const (
+	// Root URL for the Articulate Rise API.
+	defaultBaseURL = "https://rise.articulate.com"
+	// Expected host for Articulate Rise share URLs.
+	riseHost = "rise.articulate.com"
+)
+
 // shareIDRegex is compiled once at package init for extracting share IDs from URIs.
 var shareIDRegex = regexp.MustCompile(`/share/([a-zA-Z0-9_-]+)`)
 
@@ -37,7 +45,7 @@ func NewArticulateParser(logger interfaces.Logger, baseURL string, timeout time.
 		logger = NewNoOpLogger()
 	}
 	if baseURL == "" {
-		baseURL = "https://rise.articulate.com"
+		baseURL = defaultBaseURL
 	}
 	if timeout == 0 {
 		timeout = 30 * time.Second
@@ -132,7 +140,7 @@ func (p *ArticulateParser) extractShareID(uri string) (string, error) {
 	}
 
 	// Validate that it's an Articulate Rise domain
-	if parsedURL.Host != "rise.articulate.com" {
+	if parsedURL.Host != riseHost {
 		return "", fmt.Errorf("invalid domain for Articulate Rise URI: %s", parsedURL.Host)
 	}