llm: switch Skald to agent-loop Model clients; drop llm-client (phase 1, D13)

The LLM call path now runs on the agent-loop crate's clients and trait:

- core-api: BuiltLlmClient.client is Arc<dyn agent_loop::model::Model>;
  chatbot.rs (ChatbotClient + wire types) deleted; APP_NAME re-exported
  from agent-loop
- providers (openai/anthropic/ollama/openrouter/requesty/declared) build
  OpenAiModel/AnthropicModel/OllamaModel with the model's wire id
- LoggingModel decorator (llm/logging.rs) replaces LoggingChatbotClient;
  per-request correlation (session/stack/user) travels in the new
  ModelRequest.log field, never sent to providers
- llm_call/llm_loop/compactor speak Model::complete + ModelResponse;
  retriability via Model::is_retriable (structured status, B6 rule now
  the crate's default); payload persistence reads RawMeta off
  ModelResponse/ModelError
- crates/llm-client and skald-core/src/chatbot deleted

Full workspace test suite green (incl. 162 skald-core + 32 agent-loop).

crates/agent-loop/src/kernel.rs

@@ -156,6 +156,7 @@ pub(crate) async fn run(
                 conversation: params.conversation.clone(),
                 frame,
                 extras: handle.info.extras.clone(),
+                log: None,
             };
             let result = tokio::select! {
                 biased;

crates/agent-loop/src/model.rs

@@ -198,8 +198,12 @@ pub struct ModelRequest {
     pub conversation: ConversationId,
     pub frame:       FrameId,
     /// Host free-form per-request extras (e.g. reasoning knobs resolved for
-    /// this model). Merged last by the shipped clients.
+    /// this model). Merged last by the shipped clients INTO THE REQUEST BODY.
     pub extras:      Value,
+    /// Host logging/telemetry correlation (session ids, user id, …).
+    /// **Never** merged into the request body by the shipped clients — it
+    /// exists for host decorators (e.g. a `LoggingModel`) only.
+    pub log:         Option<Value>,
 }
 
 // ── Model ────────────────────────────────────────────────────────────────────