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/tools/mod.rs

@@ -1,3 +1,4 @@
+pub mod bridge;
 pub mod room_history;
 pub mod room_info;
 pub mod script;
@@ -156,6 +157,21 @@ impl ToolRegistry {
         ]
     }
 
+    /// Convert Sol's tool definitions to Mistral AgentTool format
+    /// for use with the Agents API (orchestrator agent creation).
+    pub fn agent_tool_definitions() -> Vec<mistralai_client::v1::agents::AgentTool> {
+        Self::tool_definitions()
+            .into_iter()
+            .map(|t| {
+                mistralai_client::v1::agents::AgentTool::function(
+                    t.function.name,
+                    t.function.description,
+                    t.function.parameters,
+                )
+            })
+            .collect()
+    }
+
     pub async fn execute(
         &self,
         name: &str,