feat(core): conversation history with bounded length

split llm.rs into a module with separate client and history submodules.
ConversationHistory holds an optional system prompt plus a FIFO of user/
assistant turns capped at max_turns; oldest turns evict on overflow,
system prompt is preserved across truncation and clear().

crates/jarvis-core/src/llm/mod.rs

@@ -0,0 +1,8 @@
+mod client;
+mod history;
+
+pub use client::{
+    ChatMessage, ConfigError, LlmClient, LlmError,
+    DEFAULT_BASE_URL, DEFAULT_MODEL, ENV_BASE_URL, ENV_MODEL, ENV_TOKEN,
+};
+pub use history::ConversationHistory;