feat: add multi-turn conversation history and tool execution

* feat: add multi-turn conversation history and tool execution

Major enhancement to the agent loop:

**Multi-turn conversation:**
- Add `ChatMessage` type with system/user/assistant constructors
- Add `chat_with_history` method to Provider trait (default impl
  delegates to `chat_with_system` for backward compatibility)
- Implement native `chat_with_history` on OpenRouter, Compatible,
  Reliable, and Router providers to send full message history
- Interactive mode now maintains persistent history across turns

**Tool execution:**
- Agent loop now parses `<tool_call>` XML tags from LLM responses
- Executes tools from the registry and feeds results back as
  `<tool_result>` messages
- Agentic loop continues until LLM produces final text (no tool calls)
- MAX_TOOL_ITERATIONS (10) safety limit prevents runaway loops
- System prompt includes structured tool-use protocol with JSON schemas

**Types:**
- `ChatMessage`, `ChatResponse`, `ToolCall`, `ToolResultMessage`,
  `ConversationMessage` — full conversation modeling types

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

* fix: address review comments on multi-turn + tool execution

- Add history sliding window (MAX_HISTORY_MESSAGES=50) to prevent
  unbounded conversation history growth in interactive mode
- Add 404→Responses API fallback in compatible.rs chat_with_history,
  matching chat_with_system behavior
- Use super::api_error() for error sanitization in compatible.rs
  instead of raw error body (prevents secret leakage)
- Add missing operational logs in reliable.rs chat_with_history:
  recovery, non-retryable, fallback switch warnings
- Add trim_history tests

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

* fix: address second round of review comments

- Sanitize raw error text in compatible.rs chat_with_system using
  sanitize_api_error (prevents leaking secrets in error messages)
- Add chat_with_history to MockProvider in reliable.rs tests so
  the retry/fallback path is exercised end-to-end
- Add chat_with_history_retries_then_recovers and
  chat_with_history_falls_back tests
- Log warning on malformed <tool_call> JSON instead of silent drop
- Flush stdout after print! in agent_turn so output appears before
  tool execution on line-buffered terminals
- Make interactive mode resilient to transient errors (continue
  loop instead of terminating session)

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

---------

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

src/providers/reliable.rs

@@ -1,3 +1,4 @@
+use super::traits::ChatMessage;
 use super::Provider;
 use async_trait::async_trait;
 use std::time::Duration;
@@ -121,6 +122,68 @@ impl Provider for ReliableProvider {
 
         anyhow::bail!("All providers failed. Attempts:\n{}", failures.join("\n"))
     }
+
+    async fn chat_with_history(
+        &self,
+        messages: &[ChatMessage],
+        model: &str,
+        temperature: f64,
+    ) -> anyhow::Result<String> {
+        let mut failures = Vec::new();
+
+        for (provider_name, provider) in &self.providers {
+            let mut backoff_ms = self.base_backoff_ms;
+
+            for attempt in 0..=self.max_retries {
+                match provider
+                    .chat_with_history(messages, model, temperature)
+                    .await
+                {
+                    Ok(resp) => {
+                        if attempt > 0 {
+                            tracing::info!(
+                                provider = provider_name,
+                                attempt,
+                                "Provider recovered after retries"
+                            );
+                        }
+                        return Ok(resp);
+                    }
+                    Err(e) => {
+                        let non_retryable = is_non_retryable(&e);
+                        failures.push(format!(
+                            "{provider_name} attempt {}/{}: {e}",
+                            attempt + 1,
+                            self.max_retries + 1
+                        ));
+
+                        if non_retryable {
+                            tracing::warn!(
+                                provider = provider_name,
+                                "Non-retryable error, switching provider"
+                            );
+                            break;
+                        }
+
+                        if attempt < self.max_retries {
+                            tracing::warn!(
+                                provider = provider_name,
+                                attempt = attempt + 1,
+                                max_retries = self.max_retries,
+                                "Provider call failed, retrying"
+                            );
+                            tokio::time::sleep(Duration::from_millis(backoff_ms)).await;
+                            backoff_ms = (backoff_ms.saturating_mul(2)).min(10_000);
+                        }
+                    }
+                }
+            }
+
+            tracing::warn!(provider = provider_name, "Switching to fallback provider");
+        }
+
+        anyhow::bail!("All providers failed. Attempts:\n{}", failures.join("\n"))
+    }
 }
 
 #[cfg(test)]
@@ -151,6 +214,19 @@ mod tests {
             }
             Ok(self.response.to_string())
         }
+
+        async fn chat_with_history(
+            &self,
+            _messages: &[ChatMessage],
+            _model: &str,
+            _temperature: f64,
+        ) -> anyhow::Result<String> {
+            let attempt = self.calls.fetch_add(1, Ordering::SeqCst) + 1;
+            if attempt <= self.fail_until_attempt {
+                anyhow::bail!(self.error);
+            }
+            Ok(self.response.to_string())
+        }
     }
 
     #[tokio::test]
@@ -330,4 +406,73 @@ mod tests {
         assert_eq!(primary_calls.load(Ordering::SeqCst), 1);
         assert_eq!(fallback_calls.load(Ordering::SeqCst), 1);
     }
+
+    #[tokio::test]
+    async fn chat_with_history_retries_then_recovers() {
+        let calls = Arc::new(AtomicUsize::new(0));
+        let provider = ReliableProvider::new(
+            vec![(
+                "primary".into(),
+                Box::new(MockProvider {
+                    calls: Arc::clone(&calls),
+                    fail_until_attempt: 1,
+                    response: "history ok",
+                    error: "temporary",
+                }),
+            )],
+            2,
+            1,
+        );
+
+        let messages = vec![
+            ChatMessage::system("system"),
+            ChatMessage::user("hello"),
+        ];
+        let result = provider
+            .chat_with_history(&messages, "test", 0.0)
+            .await
+            .unwrap();
+        assert_eq!(result, "history ok");
+        assert_eq!(calls.load(Ordering::SeqCst), 2);
+    }
+
+    #[tokio::test]
+    async fn chat_with_history_falls_back() {
+        let primary_calls = Arc::new(AtomicUsize::new(0));
+        let fallback_calls = Arc::new(AtomicUsize::new(0));
+
+        let provider = ReliableProvider::new(
+            vec![
+                (
+                    "primary".into(),
+                    Box::new(MockProvider {
+                        calls: Arc::clone(&primary_calls),
+                        fail_until_attempt: usize::MAX,
+                        response: "never",
+                        error: "primary down",
+                    }),
+                ),
+                (
+                    "fallback".into(),
+                    Box::new(MockProvider {
+                        calls: Arc::clone(&fallback_calls),
+                        fail_until_attempt: 0,
+                        response: "fallback ok",
+                        error: "fallback err",
+                    }),
+                ),
+            ],
+            1,
+            1,
+        );
+
+        let messages = vec![ChatMessage::user("hello")];
+        let result = provider
+            .chat_with_history(&messages, "test", 0.0)
+            .await
+            .unwrap();
+        assert_eq!(result, "fallback ok");
+        assert_eq!(primary_calls.load(Ordering::SeqCst), 2);
+        assert_eq!(fallback_calls.load(Ordering::SeqCst), 1);
+    }
 }