feat(signpost): content descriptor fields for dates, parents, authors, media and outline

- pact.ContentDescriptor gains Kind and Tools with the ContentKind and
  ContentTool constants
- pact.ContentRecord gains PublishedAt, Parents, Authors and TopHeading;
  pact.ContentLocale gains Images, Videos and Headings, with the value
  types ContentRef, ContentPerson, ContentImage, ContentVideo and
  ContentHeading; every field is optional
- TestContentDescriptorFields proves signpost hands them through and
  that a type setting none of them describes as before
- the runnable content example, the pact and signpost READMEs and the
  menu item types page document the fields

modules/signpost/example_content_test.go

@@ -4,6 +4,7 @@ import (
 	"context"
 	"fmt"
 	"strings"
+	"time"
 
 	"git.golem15.com/golem15/summercms/modules/pact"
 	"git.golem15.com/golem15/summercms/modules/signpost"
@@ -11,10 +12,13 @@ import (
 
 // Article is the example's stand-in for a content plugin's record.
 type Article struct {
-	ID        string
-	Slug      string
-	Title     string
-	Published bool
+	ID          string
+	Slug        string
+	Title       string
+	Published   bool
+	PublishedAt time.Time
+	Author      string
+	Headings    []string // the body's level-2 headings
 }
 
 // NewsPlugin is an example content plugin whose record type describes its
@@ -24,7 +28,8 @@ type NewsPlugin struct{ articles []Article }
 func (NewsPlugin) ID() string { return "acme.news" }
 
 // MenuItemTypes registers acme-article with a content descriptor: the
-// controller that edits articles, their morph type, Describe and Match.
+// controller that edits articles, their morph type, what kind of record an
+// article is, the MCP tool that edits one, Describe and Match.
 func (p NewsPlugin) MenuItemTypes() []pact.MenuItemType {
 	return []pact.MenuItemType{{
 		Key:            "acme-article",
@@ -35,6 +40,8 @@ func (p NewsPlugin) MenuItemTypes() []pact.MenuItemType {
 		Content: &pact.ContentDescriptor{
 			Controller: "acme.news.articles",
 			Model:      "acme_news_articles",
+			Kind:       pact.ContentKindArticle,
+			Tools:      map[string]string{pact.ContentToolEdit: "acme_update_article"},
 			Describe:   p.describe,
 			Match:      p.match,
 		},
@@ -46,17 +53,28 @@ func (p NewsPlugin) resolve(_ context.Context, in pact.MenuItemResolve) (*pact.R
 }
 
 // describe answers one article per locale; the URL is built from the
-// layered pattern it receives.
+// layered pattern it receives. Besides the page defaults it reports the
+// first publication, the author and the body's heading outline, whose
+// headings start at level 2 because the page renders the title as the level-1
+// heading.
 func (p NewsPlugin) describe(_ context.Context, in pact.ContentDescribe) (*pact.ContentRecord, error) {
 	for _, a := range p.articles {
 		if a.ID != in.ID {
 			continue
 		}
 		url := strings.NewReplacer(":slug", a.Slug, ":id", a.ID).Replace(in.Pattern)
+		headings := []pact.ContentHeading{}
+		for _, h := range a.Headings {
+			headings = append(headings, pact.ContentHeading{Level: 2, Text: h})
+		}
+		published := a.PublishedAt
 		return &pact.ContentRecord{
-			ID:        a.ID,
-			Published: a.Published,
-			Locales:   []pact.ContentLocale{{Locale: "en", URL: url, Title: a.Title}},
+			ID:          a.ID,
+			Published:   a.Published,
+			PublishedAt: &published,
+			Authors:     []pact.ContentPerson{{Name: a.Author}},
+			TopHeading:  2,
+			Locales:     []pact.ContentLocale{{Locale: "en", URL: url, Title: a.Title, Headings: headings}},
 		}, nil
 	}
 	return nil, nil
@@ -82,7 +100,11 @@ func (p NewsPlugin) match(_ context.Context, in pact.ContentMatch) (*pact.Conten
 
 func ExampleRegistry_MatchPath() {
 	app := exampleApp() // its menu_items.yaml overrides acme-post of BlogPlugin
-	news := NewsPlugin{articles: []Article{{ID: "7", Slug: "launch", Title: "Launch", Published: true}}}
+	news := NewsPlugin{articles: []Article{{
+		ID: "7", Slug: "launch", Title: "Launch", Published: true,
+		PublishedAt: time.Date(2026, 10, 1, 9, 0, 0, 0, time.UTC),
+		Author:      "Ada", Headings: []string{"Why", "How"},
+	}}}
 	plugins := []interface{ ID() string }{BlogPlugin{}, news}
 	if err := signpost.Activate(app, plugins); err != nil {
 		fmt.Println(err)
@@ -95,10 +117,16 @@ func ExampleRegistry_MatchPath() {
 	rec, _ := reg.Describe(context.Background(), typ.Key, "7", "")
 	fmt.Println(typ.Key, rec.Locales[0].URL)
 
+	// The descriptor's optional facts reach every consumer as given.
+	fmt.Println(typ.Content.Kind, typ.Content.Tools[pact.ContentToolEdit])
+	fmt.Println(rec.PublishedAt.Format(time.DateOnly), rec.Authors[0].Name, rec.TopHeading, len(rec.Locales[0].Headings))
+
 	// A visitor's path maps back to the record.
 	key, res, _ := reg.MatchPath(context.Background(), "/news/launch", "")
 	fmt.Println(key, res.ID, res.Exact)
 	// Output:
 	// acme-article /news/launch
+	// article acme_update_article
+	// 2026-10-01 Ada 2 2
 	// acme-article 7 true
 }