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.

config/sol.toml

@@ -2,6 +2,7 @@
 homeserver_url = "http://tuwunel.matrix.svc.cluster.local:6167"
 user_id = "@sol:sunbeam.pt"
 state_store_path = "/data/matrix-state"
+db_path = "/data/sol.db"
 
 [opensearch]
 url = "http://opensearch.data.svc.cluster.local:9200"
@@ -39,3 +40,9 @@ script_fetch_allowlist = []
 memory_extraction_enabled = true
 # evaluation_prompt_active = "custom prompt when Sol is already in conversation..."
 # evaluation_prompt_passive = "custom prompt when Sol hasn't spoken yet..."
+
+[agents]
+orchestrator_model = "mistral-medium-latest"
+domain_model = "mistral-medium-latest"
+compaction_threshold = 118000
+use_conversations_api = true