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/searxng-settings.yml

@@ -0,0 +1,25 @@
+use_default_settings: true
+server:
+  secret_key: "dev-secret-key"
+  bind_address: "0.0.0.0"
+  port: 8080
+search:
+  formats:
+    - html
+    - json
+  default_lang: "en"
+engines:
+  - name: duckduckgo
+    disabled: false
+  - name: wikipedia
+    disabled: false
+  - name: stackoverflow
+    disabled: false
+  - name: github
+    disabled: false
+  - name: google
+    disabled: true
+  - name: bing
+    disabled: true
+outgoing:
+  request_timeout: 5