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.

Cargo.toml

@@ -0,0 +1,28 @@
+[package]
+name = "sol"
+version = "0.1.0"
+edition = "2021"
+rust-version = "1.76.0"
+authors = ["Sunbeam Studios <hello@sunbeam.pt>"]
+description = "Sol — virtual librarian Matrix bot with E2EE, OpenSearch archive, and Mistral AI"
+
+[[bin]]
+name = "sol"
+path = "src/main.rs"
+
+[dependencies]
+mistralai-client = { version = "1.0.0", registry = "sunbeam" }
+matrix-sdk = { version = "0.9", features = ["e2e-encryption", "sqlite"] }
+opensearch = "2"
+tokio = { version = "1", features = ["full"] }
+serde = { version = "1", features = ["derive"] }
+serde_json = "1"
+toml = "0.8"
+tracing = "0.1"
+tracing-subscriber = { version = "0.3", features = ["env-filter"] }
+rand = "0.8"
+regex = "1"
+anyhow = "1"
+chrono = { version = "0.4", features = ["serde"] }
+url = "2"
+ruma = { version = "0.12", features = ["events", "client"] }