fix: normalize message roles for local model backends (llama.cpp, Ollama)

Local backends using strict chat templates (e.g. Mistral 3) rejected
Flynn's Anthropic-style tool_use/tool_result content blocks, causing
'roles must alternate' errors. Added getMessageTextWithTools() and
normalizeMessagesForLocal() to serialize structured blocks to plain
text, drop empty messages, and merge consecutive same-role messages.
Also fixed compaction to ensure kept messages start with user role.

src/models/media.ts

@@ -120,6 +120,78 @@ export function getMessageText(message: Message): string {
     .join('');
 }
 
+/**
+ * Serialize a message's content to a plain string, including tool_use and
+ * tool_result structured blocks that getMessageText() would discard.
+ * This is needed for local model backends (llama.cpp, Ollama) whose chat
+ * templates don't understand Anthropic-style structured content blocks.
+ */
+export function getMessageTextWithTools(message: Message): string {
+  if (typeof message.content === 'string') {
+    return message.content;
+  }
+
+  const parts: string[] = [];
+  for (const block of message.content as Record<string, unknown>[]) {
+    if (block.type === 'text' && typeof block.text === 'string') {
+      parts.push(block.text);
+    } else if (block.type === 'tool_use') {
+      const name = block.name as string;
+      let argsStr: string;
+      try {
+        argsStr = JSON.stringify(block.input);
+      } catch {
+        argsStr = String(block.input);
+      }
+      parts.push(`[Calling tool: ${name}(${argsStr})]`);
+    } else if (block.type === 'tool_result') {
+      const content = (block.content as string) ?? '';
+      const isError = block.is_error ? ' (error)' : '';
+      parts.push(`[Tool result${isError}: ${content}]`);
+    }
+  }
+  return parts.join('\n');
+}
+
+interface SimpleMessage {
+  role: 'system' | 'user' | 'assistant';
+  content: string;
+}
+
+/**
+ * Normalize a message array for local model backends that require strict
+ * role alternation (system? -> user -> assistant -> user -> ...).
+ *
+ * 1. Serializes structured tool_use/tool_result content blocks to text
+ * 2. Drops empty messages
+ * 3. Merges consecutive same-role messages with a newline separator
+ */
+export function normalizeMessagesForLocal(
+  system: string | undefined,
+  messages: Message[],
+): SimpleMessage[] {
+  const result: SimpleMessage[] = [];
+
+  if (system) {
+    result.push({ role: 'system', content: system });
+  }
+
+  for (const msg of messages) {
+    const text = getMessageTextWithTools(msg);
+    if (!text) continue; // drop empty messages
+
+    const last = result.length > 0 ? result[result.length - 1] : undefined;
+    if (last && last.role === msg.role) {
+      // Merge consecutive same-role messages
+      last.content += '\n\n' + text;
+    } else {
+      result.push({ role: msg.role, content: text });
+    }
+  }
+
+  return result;
+}
+
 /** Configuration for audio transcription via Whisper-compatible API. */
 export interface AudioTranscriptionConfig {
   /** Whisper-compatible API endpoint (e.g. "https://api.openai.com/v1/audio/transcriptions") */