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.go

@@ -0,0 +1,65 @@
+package main
+
+import (
+	"context"
+	"errors"
+
+	"git.golem15.com/golem15/summercms/internal/docsite"
+	"git.golem15.com/golem15/summercms/modules/bonfire"
+)
+
+func docsBuildCommand() bonfire.Command {
+	return bonfire.Command{
+		Name:        "docs:build",
+		Description: "Build the documentation site",
+		Flags: []bonfire.Flag{
+			{Name: "root", Description: "Repository root; src= paths and modules/ resolve against it", Default: "."},
+			{Name: "src", Description: "Docs source directory (default <root>/docs)"},
+			{Name: "out", Description: "Output directory (default <root>/site)"},
+			{Name: "base-url", Description: "Base URL for site links (overrides site.yaml base_url)"},
+			{Name: "check", Description: "Validate the docs and write nothing", Bare: true},
+		},
+		Run: func(ctx context.Context, in bonfire.Input, out bonfire.Output) error {
+			opts := docsOptions(in)
+			if flagTrue(in, "check") {
+				problems, err := docsite.Check(opts)
+				if err != nil {
+					return err
+				}
+				if len(problems) > 0 {
+					return reportDocsProblems(out, "docs:build", problems)
+				}
+				out.Printf("docs:build: no problems found\n")
+				return nil
+			}
+			result, problems, err := docsite.Build(opts)
+			if err != nil {
+				return err
+			}
+			if len(problems) > 0 {
+				return reportDocsProblems(out, "docs:build", problems)
+			}
+			out.Printf("docs:build: wrote %d pages to %s\n", result.Pages, result.Out)
+			return nil
+		},
+	}
+}
+
+func docsOptions(in bonfire.Input) docsite.Options {
+	var opts docsite.Options
+	opts.Root, _ = in.Flag("root")
+	opts.Src, _ = in.Flag("src")
+	opts.Out, _ = in.Flag("out")
+	opts.BaseURL, _ = in.Flag("base-url")
+	return opts
+}
+
+// reportDocsProblems prints one line per problem and a summary, and returns
+// a short error so the binary exits 1.
+func reportDocsProblems(out bonfire.Output, cmd string, problems []docsite.Problem) error {
+	for _, p := range problems {
+		out.Printf("%s\n", p)
+	}
+	out.Printf("%s: %d problems, nothing written\n", cmd, len(problems))
+	return errors.New(cmd + " failed")
+}