feat(11.1-01): add summer docs:build with the docsite generator core

- internal/docsite loads docs/ with strict site.yaml and frontmatter decoding
- goldmark GFM pipeline renders pages into an embedded html/template shell
- emits .html pages, .md siblings, llms.txt, llms-full.txt, search-index.json
- output guard refuses unmarked non-empty dirs and --out inside --src or root
- docs/index.md and docs/setup/installation.md; /site/ is gitignored

cmd/summer/docs_test.go

@@ -0,0 +1,59 @@
+package main
+
+import (
+	"bytes"
+	"os"
+	"path/filepath"
+	"regexp"
+	"testing"
+
+	"git.golem15.com/golem15/summercms/internal/docsite"
+	"git.golem15.com/golem15/summercms/modules/bonfire"
+)
+
+const repoRoot = "../.."
+
+// TestDocsTree fails with every problem line in the real docs tree.
+func TestDocsTree(t *testing.T) {
+	problems, err := docsite.Check(docsite.Options{Root: repoRoot})
+	if err != nil {
+		t.Fatal(err)
+	}
+	for _, p := range problems {
+		t.Error(p.String())
+	}
+}
+
+// buildRealTree runs docs:build on the real tree into a temp dir and returns
+// the output dir and the command output.
+func buildRealTree(t *testing.T) (string, string) {
+	t.Helper()
+	out := filepath.Join(t.TempDir(), "site")
+	var buf bytes.Buffer
+	root, err := bonfire.NewRoot("summer", toolCommands(), &buf)
+	if err != nil {
+		t.Fatal(err)
+	}
+	root.SetArgs([]string{"docs:build", "--root", repoRoot, "--out", out})
+	if err := root.Execute(); err != nil {
+		t.Fatalf("docs:build: %v\n%s", err, buf.String())
+	}
+	return out, buf.String()
+}
+
+func TestDocsBuildRealTree(t *testing.T) {
+	out, stdout := buildRealTree(t)
+	if !regexp.MustCompile(`docs:build: wrote \d+ pages to `).MatchString(stdout) {
+		t.Fatalf("output = %q, want a wrote-pages line", stdout)
+	}
+	for _, name := range []string{
+		"index.html", "index.md",
+		"setup/installation.html", "setup/installation.md",
+		"llms.txt", "llms-full.txt", "search-index.json",
+		"assets/site.css", docsite.MarkerFile,
+	} {
+		if _, err := os.Stat(filepath.Join(out, name)); err != nil {
+			t.Errorf("missing %s: %v", name, err)
+		}
+	}
+}