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.

Cargo.lock

@@ -1276,7 +1276,7 @@ source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "39cab71617ae0d63f51a36d69f866391735b51691dbda63cf6f96d042b63efeb"
 dependencies = [
  "libc",
- "windows-sys 0.61.2",
+ "windows-sys 0.59.0",
 ]
 
 [[package]]
@@ -2754,9 +2754,9 @@ dependencies = [
 
 [[package]]
 name = "mistralai-client"
-version = "1.0.0"
+version = "1.1.0"
 source = "sparse+https://src.sunbeam.pt/api/packages/studio/cargo/"
-checksum = "1b81079a03157d7f8342ff24a5eec16aa3d5f7660a60e4031d5ec75f6dfc911f"
+checksum = "6a17c5600508f30965a8e6ab78e947a0db7017edef8ffb022aa9ac71f50ebaff"
 dependencies = [
  "async-stream",
  "async-trait",
@@ -2810,7 +2810,7 @@ version = "0.50.3"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "7957b9740744892f114936ab4a57b3f487491bbeafaf8083688b16841a4240e5"
 dependencies = [
- "windows-sys 0.61.2",
+ "windows-sys 0.59.0",
 ]
 
 [[package]]
@@ -3316,7 +3316,7 @@ dependencies = [
  "once_cell",
  "socket2",
  "tracing",
- "windows-sys 0.52.0",
+ "windows-sys 0.59.0",
 ]
 
 [[package]]
@@ -3774,7 +3774,7 @@ dependencies = [
  "errno",
  "libc",
  "linux-raw-sys 0.4.15",
- "windows-sys 0.52.0",
+ "windows-sys 0.59.0",
 ]
 
 [[package]]
@@ -3787,7 +3787,7 @@ dependencies = [
  "errno",
  "libc",
  "linux-raw-sys 0.12.1",
- "windows-sys 0.61.2",
+ "windows-sys 0.59.0",
 ]
 
 [[package]]
@@ -4179,6 +4179,7 @@ name = "sol"
 version = "0.1.0"
 dependencies = [
  "anyhow",
+ "base64",
  "chrono",
  "deno_ast",
  "deno_core",
@@ -4191,6 +4192,7 @@ dependencies = [
  "regex",
  "reqwest",
  "ruma",
+ "rusqlite",
  "serde",
  "serde_json",
  "tempfile",
@@ -4246,7 +4248,6 @@ dependencies = [
  "cfg-if",
  "libc",
  "psm",
- "windows-sys 0.52.0",
  "windows-sys 0.59.0",
 ]
 
@@ -4795,7 +4796,7 @@ dependencies = [
  "getrandom 0.4.2",
  "once_cell",
  "rustix 1.1.4",
- "windows-sys 0.61.2",
+ "windows-sys 0.59.0",
 ]
 
 [[package]]
@@ -5661,7 +5662,7 @@ version = "0.1.11"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "c2a7b1c03c876122aa43f3020e6c3c3ee5c05081c9a00739faf7503aeba10d22"
 dependencies = [
- "windows-sys 0.61.2",
+ "windows-sys 0.59.0",
 ]
 
 [[package]]