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

Dockerfile

@@ -1,8 +1,12 @@
 FROM rust:latest AS deps
 WORKDIR /build
 
-# Copy dependency manifests and vendored crates first (cached layer)
-COPY Cargo.toml Cargo.lock ./
+# protobuf compiler for tonic-build
+RUN apt-get update && apt-get install -y protobuf-compiler && rm -rf /var/lib/apt/lists/*
+
+# Copy dependency manifests, vendored crates, and proto files first (cached layer)
+COPY Cargo.toml Cargo.lock build.rs ./
+COPY proto/ proto/
 COPY vendor/ vendor/
 
 # Set up vendored dependency resolution