story-kit: done 240_story_btw_side_question_slash_command

Implement /btw side question slash command — lets users ask quick
questions from conversation context without disrupting the main chat.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

server/src/llm/chat.rs

@@ -409,6 +409,83 @@ where
     })
 }
 
+/// Answer a one-off side question using the existing conversation as context.
+///
+/// Unlike `chat`, this function:
+/// - Does NOT perform tool calls.
+/// - Does NOT modify the main conversation history.
+/// - Does NOT touch the shared cancel signal.
+/// - Performs a single LLM call and returns the response text.
+pub async fn side_question<U>(
+    context_messages: Vec<Message>,
+    question: String,
+    config: ProviderConfig,
+    store: &dyn StoreOps,
+    mut on_token: U,
+) -> Result<String, String>
+where
+    U: FnMut(&str) + Send,
+{
+    use crate::llm::providers::anthropic::AnthropicProvider;
+    use crate::llm::providers::ollama::OllamaProvider;
+
+    // Use a local cancel channel that is never cancelled, so the side question
+    // runs to completion independently of any main chat cancel signal.
+    // Keep `_cancel_tx` alive for the duration of the function so the channel
+    // stays open and `changed()` inside the providers does not spuriously fire.
+    let (_cancel_tx, cancel_rx) = tokio::sync::watch::channel(false);
+    let mut cancel_rx = cancel_rx;
+    cancel_rx.borrow_and_update();
+
+    let base_url = config
+        .base_url
+        .clone()
+        .unwrap_or_else(|| "http://localhost:11434".to_string());
+
+    let is_claude_code = config.provider == "claude-code";
+    let is_claude = !is_claude_code && config.model.starts_with("claude-");
+
+    // Build a minimal history: existing context + the side question.
+    let mut history = context_messages;
+    history.push(Message {
+        role: Role::User,
+        content: question,
+        tool_calls: None,
+        tool_call_id: None,
+    });
+
+    // No tools for side questions.
+    let tools: &[ToolDefinition] = &[];
+
+    let response = if is_claude {
+        let api_key = get_anthropic_api_key_impl(store)?;
+        let provider = AnthropicProvider::new(api_key);
+        provider
+            .chat_stream(
+                &config.model,
+                &history,
+                tools,
+                &mut cancel_rx,
+                |token| on_token(token),
+                |_tool_name| {},
+            )
+            .await
+            .map_err(|e| format!("Anthropic Error: {e}"))?
+    } else if is_claude_code {
+        return Err("Claude Code provider does not support side questions".to_string());
+    } else {
+        let provider = OllamaProvider::new(base_url);
+        provider
+            .chat_stream(&config.model, &history, tools, &mut cancel_rx, |token| {
+                on_token(token)
+            })
+            .await
+            .map_err(|e| format!("Ollama Error: {e}"))?
+    };
+
+    Ok(response.content.unwrap_or_default())
+}
+
 async fn execute_tool(call: &ToolCall, state: &SessionState) -> String {
     use crate::io::{fs, search, shell};