feat: initial Sol virtual librarian implementation

Matrix bot with E2EE (matrix-sdk 0.9) that passively archives all
messages to OpenSearch and responds to queries via Mistral AI with
function calling tools.

Core systems:
- Archive: bulk OpenSearch indexer with batch/flush, edit/redaction
  handling, embedding pipeline passthrough
- Brain: rule-based engagement evaluator (mentions, DMs, name
  invocations), LLM-powered spontaneous engagement, per-room
  conversation context windows, response delay simulation
- Tools: search_archive, get_room_context, list_rooms, get_room_members
  registered as Mistral function calling tools with iterative tool loop
- Personality: templated system prompt with Sol's librarian persona

47 unit tests covering config, evaluator, conversation windowing,
personality templates, schema serialization, and search query building.

config/sol.toml

@@ -0,0 +1,28 @@
+[matrix]
+homeserver_url = "http://tuwunel.matrix.svc.cluster.local:6167"
+user_id = "@sol:sunbeam.pt"
+state_store_path = "/data/matrix-state"
+
+[opensearch]
+url = "http://opensearch.data.svc.cluster.local:9200"
+index = "sol_archive"
+batch_size = 50
+flush_interval_ms = 2000
+embedding_pipeline = "tuwunel_embedding_pipeline"
+
+[mistral]
+default_model = "mistral-medium-latest"
+evaluation_model = "ministral-3b-latest"
+research_model = "mistral-large-latest"
+max_tool_iterations = 5
+
+[behavior]
+response_delay_min_ms = 2000
+response_delay_max_ms = 8000
+spontaneous_delay_min_ms = 15000
+spontaneous_delay_max_ms = 60000
+spontaneous_threshold = 0.7
+room_context_window = 30
+dm_context_window = 100
+backfill_on_join = true
+backfill_limit = 10000