feat: add website builder using goldmark (replaces build.js)

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

internal/builder/builder.go

@@ -0,0 +1,51 @@
+package builder
+
+import (
+	"bytes"
+	"fmt"
+	"os"
+	"strings"
+
+	"github.com/yuin/goldmark"
+	"github.com/yuin/goldmark/extension"
+	"github.com/yuin/goldmark/renderer/html"
+)
+
+// Build converts a Markdown file to HTML using a template.
+// The template must contain a placeholder element that will be replaced with the rendered content.
+func Build(markdownPath, templatePath, outputPath string) error {
+	md, err := os.ReadFile(markdownPath)
+	if err != nil {
+		return fmt.Errorf("read markdown: %w", err)
+	}
+
+	tmpl, err := os.ReadFile(templatePath)
+	if err != nil {
+		return fmt.Errorf("read template: %w", err)
+	}
+
+	// Convert markdown to HTML
+	gm := goldmark.New(
+		goldmark.WithExtensions(extension.GFM),
+		goldmark.WithRendererOptions(html.WithUnsafe()),
+	)
+	var buf bytes.Buffer
+	if err := gm.Convert(md, &buf); err != nil {
+		return fmt.Errorf("convert markdown: %w", err)
+	}
+
+	// Inject into template — support both placeholder formats
+	output := string(tmpl)
+	replacements := []struct{ old, new string }{
+		{`<div id="md"></div>`, `<div id="md">` + buf.String() + `</div>`},
+		{`<section id="md" class="main-content"></section>`, `<section id="md" class="main-content">` + buf.String() + `</section>`},
+	}
+	for _, r := range replacements {
+		output = strings.Replace(output, r.old, r.new, 1)
+	}
+
+	if err := os.WriteFile(outputPath, []byte(output), 0644); err != nil {
+		return fmt.Errorf("write output: %w", err)
+	}
+	return nil
+}