feat(grpc): dev mode, agent prefix, system prompt, error UX

- gRPC dev_mode config: disables JWT auth, uses fixed dev identity
- Agent prefix (agents.agent_prefix): dev agents use "dev-sol-orchestrator"
  to avoid colliding with production on shared Mistral accounts
- Coding sessions use instructions (system prompt + coding addendum)
  with mistral-medium-latest for personality adherence
- Conversations API: don't send both model + agent_id (422 fix)
- GrpcState carries system_prompt + orchestrator_agent_id
- Session.end() keeps session active for reuse (not "ended")
- User messages posted as m.notice, assistant as m.text (role detection)
- History loaded from Matrix room on session resume
- Docker Compose local dev stack: OpenSearch 3 + Tuwunel + SearXNG
- Dev config: localhost URLs, dev_mode, opensearch-init.sh for ML setup

dev/Dockerfile

@@ -0,0 +1,26 @@
+## Dev Dockerfile — builds for the host platform (no cross-compilation).
+FROM rust:latest AS deps
+WORKDIR /build
+
+RUN apt-get update && apt-get install -y protobuf-compiler && rm -rf /var/lib/apt/lists/*
+
+COPY Cargo.toml Cargo.lock build.rs ./
+COPY proto/ proto/
+COPY vendor/ vendor/
+
+RUN mkdir -p .cargo && \
+    printf '[registries.sunbeam]\nindex = "sparse+https://src.sunbeam.pt/api/packages/studio/cargo/"\n\n[source.crates-io]\nreplace-with = "vendored-sources"\n\n[source."sparse+https://src.sunbeam.pt/api/packages/studio/cargo/"]\nregistry = "sparse+https://src.sunbeam.pt/api/packages/studio/cargo/"\nreplace-with = "vendored-sources"\n\n[source.vendored-sources]\ndirectory = "vendor/"\n' \
+    > .cargo/config.toml
+
+RUN mkdir -p src && echo "fn main(){}" > src/main.rs && \
+    cargo build --release && \
+    rm src/main.rs && rm target/release/sol
+
+FROM deps AS builder
+COPY src/ src/
+RUN find src/ -name '*.rs' -exec touch {} + && \
+    cargo build --release
+
+FROM gcr.io/distroless/cc-debian12:nonroot
+COPY --from=builder /build/target/release/sol /
+ENTRYPOINT ["/sol"]