feat(inbound): emit media in notification instead of appending footer

Channel plugins now have a structured way to propagate attachments —
Plexum's host turns each MediaItem into a canonical.ImageBlock content
on the user message. This replaces the F-6 workaround that crammed
local file paths into a markdown footer at the end of the message
text.

internal/inbound/inbound.go:
- Notifier signature gains `media []MediaItem`; MediaItem mirrors
  the host's channel.MediaRef (Path/MediaType/Name)
- dispatch downloads attachments (unchanged) then forwards results
  as MediaItem to Notify — no more footer appending
- dispatch now also receives guildEndpoint so it can resolve Fabric's
  RELATIVE attachment URLs (`/api/files/<id>`) against the guild base.
  Previously the downloader received the relative path verbatim and
  failed every fetch silently.

cmd/plexum-fabric-channel-plugin/main.go:
- notifier closure pushes media[] in the EmitNotification payload

Live verified: alice uploads blue 32x32 PNG → Fabric guild → plugin
downloads to /tmp/plexum-fabric/<msg>/blue32.png → emits inbound with
media → host routes to kimi agent → Kimi: "Blue".

(MiniMax M2.7 is text-only per openclaw model definitions, so the
same flow against MiniMax returns "I don't see an image" — that's a
model capability limit, not a plugin issue.)

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

cmd/plexum-fabric-channel-plugin/main.go

@@ -201,12 +201,24 @@ func (p *fabricPlugin) Init(ctx context.Context, host plugin.HostAPI) error {
 		ctxBg, cancel := context.WithCancel(context.Background())
 		p.inboundCancel = cancel
 		p.inboundDone = make(chan struct{})
-		notifier := func(channelName, message, sessionID string) {
-			p.host.EmitNotification("notifications/plexum/channel/inbound", map[string]any{
+		notifier := func(channelName, message, sessionID string, media []inbound.MediaItem) {
+			payload := map[string]any{
 				"channel_name": channelName,
 				"message":      message,
 				"session_id":   sessionID,
-			})
+			}
+			if len(media) > 0 {
+				mediaWire := make([]map[string]any, 0, len(media))
+				for _, m := range media {
+					mediaWire = append(mediaWire, map[string]any{
+						"path":       m.Path,
+						"media_type": m.MediaType,
+						"name":       m.Name,
+					})
+				}
+				payload["media"] = mediaWire
+			}
+			p.host.EmitNotification("notifications/plexum/channel/inbound", payload)
 		}
 		// slog wrapping plugin.HostAPI.Log isn't worth the indirection
 		// here; use a discard-style adapter that pipes WARN/INFO to