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/skald-core/src/db/llm_requests/mod.rs

@@ -1,7 +1,7 @@
 //! DB operations for the `llm_requests` table (metadata only).
 //!
 //! Every `chat_with_tools` call is logged here by the
-//! [`crate::chatbot::logging::LoggingChatbotClient`] wrapper.
+//! [`crate::llm::logging::LoggingModel`] decorator.
 //! Payloads (request/response bodies + headers) live in `llm_request_payloads`
 //! in the owner bucket (`{userid}.db`), correlated by `request_id`.
 //! Rows are retained for `llm.request_log.retention_days` days (default 14).