feat: private-message send via POST /posts.json targets

Client.SendMessage(title, raw, targets) creates a private-message
topic by posting to /posts.json with target_usernames /
target_group_names / target_emails (comma-joined on the wire, as
Discourse expects, never JSON arrays) instead of a category. The
result reuses the topic ids type, so URL() renders the PM link.
Local validation requires title, body and at least one target;
undeliverable recipients surface as the typed ErrForbidden class.
Fake-server tests assert the wire shape and the recorded recipients.

Part of Redmine 507 (Discourse Go client): PM delivery is how the
495 pipeline pings humans that a briefing has landed.

discourse_test.go

@@ -5,6 +5,7 @@ import (
 	"encoding/json"
 	"errors"
 	"fmt"
+	"io"
 	"net/http"
 	"net/http/httptest"
 	"strings"
@@ -28,6 +29,8 @@ type fakeDiscourse struct {
 	topics     map[int]topicRec
 	nextT      int
 	searchFail int
+	pms        []pmRec // private messages, in creation order
+	lastBody   string  // raw JSON body of the last POST /posts.json
 	srv        *httptest.Server
 }
 
@@ -43,6 +46,13 @@ type topicRec struct {
 	Cat   int
 }
 
+type pmRec struct {
+	TopicID int
+	Title   string
+	Targets []string
+	Raw     string
+}
+
 func newFake(t *testing.T) *fakeDiscourse {
 	t.Helper()
 	f := &fakeDiscourse{
@@ -111,13 +121,47 @@ func newFake(t *testing.T) *fakeDiscourse {
 			writeJSON(w, 405, nil)
 			return
 		}
-		var req CreatePostRequest
-		if err := json.NewDecoder(r.Body).Decode(&req); err != nil {
+		body, err := io.ReadAll(r.Body)
+		if err != nil {
+			writeJSON(w, 400, map[string]any{"errors": []string{"bad body"}})
+			return
+		}
+		var req struct {
+			CreatePostRequest
+			TargetUsernames  string `json:"target_usernames"`
+			TargetGroupNames string `json:"target_group_names"`
+			TargetEmails     string `json:"target_emails"`
+		}
+		if err := json.Unmarshal(body, &req); err != nil {
 			writeJSON(w, 400, map[string]any{"errors": []string{"bad json"}})
 			return
 		}
 		f.mu.Lock()
 		defer f.mu.Unlock()
+		f.lastBody = string(body)
+		if req.TargetUsernames != "" || req.TargetGroupNames != "" || req.TargetEmails != "" {
+			// private-message creation
+			targets := splitCSV(req.TargetUsernames)
+			for _, name := range append(append(targets, splitCSV(req.TargetGroupNames)...), splitCSV(req.TargetEmails)...) {
+				if strings.HasPrefix(name, "denied") {
+					writeJSON(w, 403, map[string]any{"errors": []string{"You are not permitted to view the requested resource."}})
+					return
+				}
+			}
+			if strings.TrimSpace(req.Title) == "" {
+				writeJSON(w, 422, map[string]any{"errors": []string{"Title can't be blank"}})
+				return
+			}
+			tid := f.nextT
+			f.nextT++
+			slug := strings.ToLower(strings.ReplaceAll(req.Title, " ", "-"))
+			f.pms = append(f.pms, pmRec{TopicID: tid, Title: req.Title, Targets: targets, Raw: req.Raw})
+			pid := f.nextPost
+			f.nextPost++
+			f.posts[pid] = postRec{Post: Post{ID: pid, TopicID: tid, PostNumber: 1, Raw: req.Raw}}
+			writeJSON(w, 200, map[string]any{"id": pid, "topic_id": tid, "topic_slug": slug})
+			return
+		}
 		if req.TopicID == 0 { // topic creation
 			if strings.TrimSpace(req.Title) == "" || req.Category == 0 {
 				writeJSON(w, 422, map[string]any{"errors": []string{"Title can't be blank"}})
@@ -264,6 +308,21 @@ func writeJSON(w http.ResponseWriter, code int, body any) {
 	_ = json.NewEncoder(w).Encode(body)
 }
 
+// splitCSV splits a comma-joined target list ("ops,reachableceo").
+func splitCSV(s string) []string {
+	if s == "" {
+		return nil
+	}
+	parts := strings.Split(s, ",")
+	out := parts[:0]
+	for _, p := range parts {
+		if p = strings.TrimSpace(p); p != "" {
+			out = append(out, p)
+		}
+	}
+	return out
+}
+
 func testClient(t *testing.T, f *fakeDiscourse) *Client {
 	t.Helper()
 	c, err := New(f.srv.URL, testKey, "")