feat: multi-agent architecture with Conversations API and persistent state

Mistral Agents + Conversations API integration:
- Orchestrator agent created on startup with Sol's personality + tools
- ConversationRegistry routes messages through persistent conversations
- Per-room conversation state (room_id → conversation_id + token counts)
- Function call handling within conversation responses
- Configurable via [agents] section in sol.toml (use_conversations_api flag)

Multimodal support:
- m.image detection and Matrix media download (mxc:// → base64 data URI)
- ContentPart-based messages sent to Mistral vision models
- Archive stores media_urls for image messages

System prompt rewrite:
- 687 → 150 lines — dense, few-shot examples, hard rules
- {room_context_rules} placeholder for group vs DM behavior
- Sender prefixing (<@user:server>) for multi-user turns in group rooms

SQLite persistence (/data/sol.db):
- Conversation mappings and agent IDs survive reboots
- WAL mode for concurrent reads
- Falls back to in-memory on failure (sneezes into all rooms to signal)
- PVC already mounted at /data alongside Matrix SDK state store

New modules:
- src/persistence.rs — SQLite state store
- src/conversations.rs — ConversationRegistry + message merging
- src/agents/{mod,definitions,registry}.rs — agent lifecycle
- src/agent_ux.rs — reaction + thread progress UX
- src/tools/bridge.rs — tool dispatch for domain agents

102 tests passing.

src/matrix_utils.rs

@@ -1,3 +1,4 @@
+use matrix_sdk::media::{MediaFormat, MediaRequestParameters};
 use matrix_sdk::room::Room;
 use matrix_sdk::RoomMemberships;
 use ruma::events::room::message::{
@@ -67,6 +68,61 @@ pub async fn send_reaction(
     Ok(())
 }
 
+/// Extract image info from an m.image message event.
+/// Returns (mxc_url, mimetype, body/caption) if present.
+pub fn extract_image(event: &OriginalSyncRoomMessageEvent) -> Option<(String, String, String)> {
+    if let MessageType::Image(image) = &event.content.msgtype {
+        let url = match &image.source {
+            ruma::events::room::MediaSource::Plain(mxc) => mxc.to_string(),
+            ruma::events::room::MediaSource::Encrypted(_) => return None,
+        };
+        let mime = image
+            .info
+            .as_ref()
+            .and_then(|i| i.mimetype.clone())
+            .unwrap_or_else(|| "image/png".to_string());
+        let caption = image.body.clone();
+        Some((url, mime, caption))
+    } else {
+        None
+    }
+}
+
+/// Download image bytes from a Matrix mxc:// URL via the media API.
+/// Returns the raw bytes as a base64 data URI suitable for Mistral vision.
+pub async fn download_image_as_data_uri(
+    client: &matrix_sdk::Client,
+    event: &OriginalSyncRoomMessageEvent,
+) -> Option<String> {
+    if let MessageType::Image(image) = &event.content.msgtype {
+        let media_source = &image.source;
+        let mime = image
+            .info
+            .as_ref()
+            .and_then(|i| i.mimetype.clone())
+            .unwrap_or_else(|| "image/png".to_string());
+
+        let request = MediaRequestParameters {
+            source: media_source.clone(),
+            format: MediaFormat::File,
+        };
+
+        match client.media().get_media_content(&request, true).await {
+            Ok(bytes) => {
+                use base64::Engine;
+                let b64 = base64::engine::general_purpose::STANDARD.encode(&bytes);
+                Some(format!("data:{};base64,{}", mime, b64))
+            }
+            Err(e) => {
+                tracing::warn!("Failed to download image: {e}");
+                None
+            }
+        }
+    } else {
+        None
+    }
+}
+
 /// Get the display name for a room.
 pub fn room_display_name(room: &Room) -> String {
     room.cached_display_name()