feat: per-user auto-memory with ResponseContext

Three memory channels: hidden tool (sol.memory.set/get in scripts),
pre-response injection (relevant memories loaded into system prompt),
and post-response extraction (ministral-3b extracts facts after each
response). User isolation enforced at Rust level — user_id derived
from Matrix sender, never from script arguments.

New modules: context (ResponseContext), memory (schema, store, extractor).
ResponseContext threaded through responder → tools → script runtime.
OpenSearch index sol_user_memory created on startup alongside archive.

src/matrix_utils.rs

@@ -3,7 +3,8 @@ use matrix_sdk::RoomMemberships;
 use ruma::events::room::message::{
     MessageType, OriginalSyncRoomMessageEvent, Relation, RoomMessageEventContent,
 };
-use ruma::events::relation::InReplyTo;
+use ruma::events::relation::{Annotation, InReplyTo};
+use ruma::events::reaction::ReactionEventContent;
 use ruma::OwnedEventId;
 
 /// Extract the plain-text body from a message event.
@@ -45,15 +46,27 @@ pub fn extract_thread_id(event: &OriginalSyncRoomMessageEvent) -> Option<OwnedEv
     None
 }
 
-/// Build a reply message content with m.in_reply_to relation.
+/// Build a reply message content with m.in_reply_to relation and markdown rendering.
 pub fn make_reply_content(body: &str, reply_to_event_id: OwnedEventId) -> RoomMessageEventContent {
-    let mut content = RoomMessageEventContent::text_plain(body);
+    let mut content = RoomMessageEventContent::text_markdown(body);
     content.relates_to = Some(Relation::Reply {
         in_reply_to: InReplyTo::new(reply_to_event_id),
     });
     content
 }
 
+/// Send an emoji reaction to a message.
+pub async fn send_reaction(
+    room: &Room,
+    event_id: OwnedEventId,
+    emoji: &str,
+) -> anyhow::Result<()> {
+    let annotation = Annotation::new(event_id, emoji.to_string());
+    let content = ReactionEventContent::new(annotation);
+    room.send(content).await?;
+    Ok(())
+}
+
 /// Get the display name for a room.
 pub fn room_display_name(room: &Room) -> String {
     room.cached_display_name()