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/router.rs

@@ -1,3 +1,4 @@
+use super::traits::ChatMessage;
 use super::Provider;
 use async_trait::async_trait;
 use std::collections::HashMap;
@@ -112,6 +113,19 @@ impl Provider for RouterProvider {
             .await
     }
 
+    async fn chat_with_history(
+        &self,
+        messages: &[ChatMessage],
+        model: &str,
+        temperature: f64,
+    ) -> anyhow::Result<String> {
+        let (provider_idx, resolved_model) = self.resolve(model);
+        let (_, provider) = &self.providers[provider_idx];
+        provider
+            .chat_with_history(messages, &resolved_model, temperature)
+            .await
+    }
+
     async fn warmup(&self) -> anyhow::Result<()> {
         for (name, provider) in &self.providers {
             tracing::info!(provider = name, "Warming up routed provider");