!explain via ❓ reaction: thread-reply with a 3-bullet TL;DR

When a user reacts ❓ on one of Pete's posts, fetch the article body,
ask Ollama for a 3-bullet summary, and post it as a threaded reply
rooted at the original story event. Per-process cooldown of 5min per
story keeps repeated reactions from re-summarizing.

- ingestion.FetchArticleBody: visible <p> text capped at 8000 chars
- classifier.OllamaClient.GenerateText: non-JSON variant
- storage.GetStoryByGUID: full row lookup
- matrix.PostThreadedReply: m.thread + m.in_reply_to fallback
- poster.SetReactionCallback: optional hook fired after recording
- New package: internal/explainer

internal/matrix/client.go

@@ -294,6 +294,35 @@ func (c *Client) Stop() {
 	}
 }
 
+// PostThreadedReply sends a plain/HTML message into the thread rooted at
+// rootEventID. Resolves channel name → room ID like PostStory. The is_falling_back
+// flag plus m.in_reply_to gives non-thread-aware clients a sensible quoted reply.
+func (c *Client) PostThreadedReply(channel string, rootEventID id.EventID, plain, htmlBody string) error {
+	roomID, ok := c.channels[channel]
+	if !ok {
+		return fmt.Errorf("unknown channel: %s", channel)
+	}
+	ctx, cancel := context.WithTimeout(context.Background(), 30*time.Second)
+	defer cancel()
+
+	content := &event.MessageEventContent{
+		MsgType:       event.MsgText,
+		Body:          plain,
+		Format:        event.FormatHTML,
+		FormattedBody: htmlBody,
+		RelatesTo: &event.RelatesTo{
+			Type:    event.RelThread,
+			EventID: rootEventID,
+			InReplyTo: &event.InReplyTo{
+				EventID: rootEventID,
+			},
+			IsFallingBack: true,
+		},
+	}
+	_, err := c.mx.SendMessageEvent(ctx, roomID, event.EventMessage, content)
+	return err
+}
+
 // PostStory sends a story to a channel. Returns the text event ID for reaction tracking.
 // If the story has a validated image, it's uploaded and sent as a separate m.image event first.
 func (c *Client) PostStory(channel string, story *PostableStory) (id.EventID, error) {