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.

Dockerfile

@@ -0,0 +1,14 @@
+FROM rust:1.86 AS builder
+WORKDIR /build
+
+# Configure Sunbeam Cargo registry (Gitea) — anonymous read access
+RUN mkdir -p /usr/local/cargo/registry && \
+    printf '[registries.sunbeam]\nindex = "sparse+https://src.sunbeam.pt/api/packages/studio/cargo/"\n' \
+    >> /usr/local/cargo/config.toml
+
+COPY . .
+RUN cargo build --release --target x86_64-unknown-linux-gnu
+
+FROM gcr.io/distroless/cc-debian12:nonroot
+COPY --from=builder /build/target/x86_64-unknown-linux-gnu/release/sol /
+ENTRYPOINT ["/sol"]