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/openrouter.rs

@@ -1,4 +1,4 @@
-use crate::providers::traits::Provider;
+use crate::providers::traits::{ChatMessage, Provider};
 use async_trait::async_trait;
 use reqwest::Client;
 use serde::{Deserialize, Serialize};
@@ -22,7 +22,7 @@ struct Message {
 }
 
 #[derive(Debug, Deserialize)]
-struct ChatResponse {
+struct ApiChatResponse {
     choices: Vec<Choice>,
 }
 
@@ -112,7 +112,57 @@ impl Provider for OpenRouterProvider {
             return Err(super::api_error("OpenRouter", response).await);
         }
 
-        let chat_response: ChatResponse = response.json().await?;
+        let chat_response: ApiChatResponse = response.json().await?;
+
+        chat_response
+            .choices
+            .into_iter()
+            .next()
+            .map(|c| c.message.content)
+            .ok_or_else(|| anyhow::anyhow!("No response from OpenRouter"))
+    }
+
+    async fn chat_with_history(
+        &self,
+        messages: &[ChatMessage],
+        model: &str,
+        temperature: f64,
+    ) -> anyhow::Result<String> {
+        let api_key = self.api_key.as_ref()
+            .ok_or_else(|| anyhow::anyhow!("OpenRouter API key not set. Run `zeroclaw onboard` or set OPENROUTER_API_KEY env var."))?;
+
+        let api_messages: Vec<Message> = messages
+            .iter()
+            .map(|m| Message {
+                role: m.role.clone(),
+                content: m.content.clone(),
+            })
+            .collect();
+
+        let request = ChatRequest {
+            model: model.to_string(),
+            messages: api_messages,
+            temperature,
+        };
+
+        let response = self
+            .client
+            .post("https://openrouter.ai/api/v1/chat/completions")
+            .header("Authorization", format!("Bearer {api_key}"))
+            .header(
+                "HTTP-Referer",
+                "https://github.com/theonlyhennygod/zeroclaw",
+            )
+            .header("X-Title", "ZeroClaw")
+            .json(&request)
+            .send()
+            .await?;
+
+        if !response.status().is_success() {
+            return Err(super::api_error("OpenRouter", response).await);
+        }
+
+        let chat_response: ApiChatResponse = response.json().await?;
 
         chat_response
             .choices