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/history.rs

@@ -0,0 +1,115 @@
+use super::client::ChatMessage;
+
+pub struct ConversationHistory {
+    system: Option<ChatMessage>,
+    turns: Vec<ChatMessage>,
+    max_turns: usize,
+}
+
+impl ConversationHistory {
+    pub fn new(system_prompt: impl Into<String>, max_turns: usize) -> Self {
+        Self {
+            system: Some(ChatMessage::system(system_prompt)),
+            turns: Vec::new(),
+            max_turns: max_turns.max(1),
+        }
+    }
+
+    pub fn without_system(max_turns: usize) -> Self {
+        Self {
+            system: None,
+            turns: Vec::new(),
+            max_turns: max_turns.max(1),
+        }
+    }
+
+    pub fn push_user(&mut self, content: impl Into<String>) {
+        self.turns.push(ChatMessage::user(content));
+        self.truncate();
+    }
+
+    pub fn push_assistant(&mut self, content: impl Into<String>) {
+        self.turns.push(ChatMessage::assistant(content));
+        self.truncate();
+    }
+
+    pub fn snapshot(&self) -> Vec<ChatMessage> {
+        let mut out = Vec::with_capacity(self.turns.len() + 1);
+        if let Some(s) = &self.system {
+            out.push(s.clone());
+        }
+        out.extend(self.turns.iter().cloned());
+        out
+    }
+
+    pub fn turns(&self) -> &[ChatMessage] {
+        &self.turns
+    }
+
+    pub fn clear(&mut self) {
+        self.turns.clear();
+    }
+
+    fn truncate(&mut self) {
+        if self.turns.len() > self.max_turns {
+            let drop = self.turns.len() - self.max_turns;
+            self.turns.drain(0..drop);
+        }
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn evicts_oldest_turns_but_keeps_system_prompt() {
+        let mut h = ConversationHistory::new("you are jarvis", 4);
+        h.push_user("u1");
+        h.push_assistant("a1");
+        h.push_user("u2");
+        h.push_assistant("a2");
+        h.push_user("u3");
+
+        let snap = h.snapshot();
+        assert_eq!(snap[0].role, "system");
+        assert_eq!(snap[0].content, "you are jarvis");
+        assert_eq!(snap.len(), 1 + 4);
+
+        let contents: Vec<&str> = snap.iter().skip(1).map(|m| m.content.as_str()).collect();
+        assert_eq!(contents, vec!["a1", "u2", "a2", "u3"]);
+    }
+
+    #[test]
+    fn snapshot_with_no_system_returns_only_turns() {
+        let mut h = ConversationHistory::without_system(3);
+        h.push_user("hi");
+        h.push_assistant("hello");
+        let snap = h.snapshot();
+        assert_eq!(snap.len(), 2);
+        assert_eq!(snap[0].role, "user");
+        assert_eq!(snap[1].role, "assistant");
+    }
+
+    #[test]
+    fn cap_of_zero_is_clamped_to_one() {
+        let mut h = ConversationHistory::new("sys", 0);
+        h.push_user("a");
+        h.push_user("b");
+        let snap = h.snapshot();
+        assert_eq!(snap.len(), 2);
+        assert_eq!(snap[0].role, "system");
+        assert_eq!(snap[1].content, "b");
+    }
+
+    #[test]
+    fn clear_removes_turns_but_not_system() {
+        let mut h = ConversationHistory::new("sys", 4);
+        h.push_user("u");
+        h.push_assistant("a");
+        h.clear();
+        let snap = h.snapshot();
+        assert_eq!(snap.len(), 1);
+        assert_eq!(snap[0].role, "system");
+    }
+}

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;