fix(channels): use canonical IMAGE marker in Matrix channel

Matrix image messages used lowercase `[image: ...]` format instead of
the canonical `[IMAGE:...]` marker used by all other channels (Telegram,
Slack, Discord, QQ, LinQ). This caused Matrix image attachments to
bypass the multimodal vision pipeline which looks for `[IMAGE:...]`.

Closes #3486

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/channels/matrix.rs

@@ -746,7 +746,7 @@ impl Channel for MatrixChannel {
                     MessageType::Notice(content) => (content.body.clone(), None),
                     MessageType::Image(content) => {
                         let dl = media_info(&content.source, &content.body);
-                        (format!("[image: {}]", content.body), dl)
+                        (format!("[IMAGE:{}]", content.body), dl)
                     }
                     MessageType::File(content) => {
                         let dl = media_info(&content.source, &content.body);