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

src/config.rs

@@ -45,6 +45,9 @@ pub struct AgentsConfig {
     /// Model for coding agent sessions (sunbeam code).
     #[serde(default = "default_coding_model")]
     pub coding_model: String,
+    /// Agent name prefix — set to "dev" in local dev to avoid colliding with production agents.
+    #[serde(default)]
+    pub agent_prefix: String,
 }
 
 impl Default for AgentsConfig {
@@ -59,6 +62,7 @@ impl Default for AgentsConfig {
             research_max_agents: default_research_max_agents(),
             research_max_depth: default_research_max_depth(),
             coding_model: default_coding_model(),
+            agent_prefix: String::new(),
         }
     }
 }
@@ -239,16 +243,19 @@ fn default_research_agent_model() -> String { "ministral-3b-latest".into() }
 fn default_research_max_iterations() -> usize { 10 }
 fn default_research_max_agents() -> usize { 25 }
 fn default_research_max_depth() -> usize { 4 }
-fn default_coding_model() -> String { "devstral-small-2506".into() }
+fn default_coding_model() -> String { "mistral-medium-latest".into() }
 
 #[derive(Debug, Clone, Deserialize)]
 pub struct GrpcConfig {
     /// Address to listen on (default: 0.0.0.0:50051).
     #[serde(default = "default_grpc_addr")]
     pub listen_addr: String,
-    /// JWKS URL for JWT validation (default: Hydra's .well-known endpoint).
+    /// JWKS URL for JWT validation. Required unless dev_mode is true.
     #[serde(default)]
     pub jwks_url: Option<String>,
+    /// Dev mode: disables JWT auth, uses a fixed dev identity.
+    #[serde(default)]
+    pub dev_mode: bool,
 }
 
 fn default_grpc_addr() -> String { "0.0.0.0:50051".into() }