fix(agent): detect repeated tool call loops and make max_iterations configurable

Local LLMs often get stuck calling the same tool repeatedly because they
lack the sophistication to synthesize results. The agent loop had no
safeguard — it re-executed whatever the model requested up to 10 times.

Add fingerprint-based loop detection: if the same tool+args combination
repeats 3 consecutive times, break the loop and return the last results.
Also add agents.max_iterations to the config schema so the iteration
limit is user-configurable (default: 10).

src/backends/native/agent.test.ts

@@ -121,14 +121,18 @@ describe('NativeAgent tool loop', () => {
     expect(mockClient.chat).toHaveBeenCalledTimes(2);
   });
 
-  it('respects max iterations', async () => {
-    // Model always returns tool_use
+  it('respects max iterations when tool calls vary', async () => {
+    // Model always returns tool_use but with different args each time (no loop detection)
+    let callCount = 0;
     const mockClient: ModelClient = {
-      chat: vi.fn().mockResolvedValue({
-        content: '',
-        stopReason: 'tool_use',
-        usage: { inputTokens: 10, outputTokens: 5 },
-        toolCalls: [{ id: 'call_1', name: 'test.echo', args: { text: 'loop' } }],
+      chat: vi.fn().mockImplementation(() => {
+        callCount++;
+        return {
+          content: '',
+          stopReason: 'tool_use',
+          usage: { inputTokens: 10, outputTokens: 5 },
+          toolCalls: [{ id: `call_${callCount}`, name: 'test.echo', args: { text: `attempt_${callCount}` } }],
+        };
       }),
     };
 
@@ -150,6 +154,37 @@ describe('NativeAgent tool loop', () => {
     expect(mockClient.chat).toHaveBeenCalledTimes(3);
   });
 
+  it('detects repeated identical tool calls and breaks the loop', async () => {
+    // Model always returns the exact same tool call — simulates local LLM stuck in a loop
+    const mockClient: ModelClient = {
+      chat: vi.fn().mockResolvedValue({
+        content: '',
+        stopReason: 'tool_use',
+        usage: { inputTokens: 10, outputTokens: 5 },
+        toolCalls: [{ id: 'call_1', name: 'test.echo', args: { text: 'same thing' } }],
+      }),
+    };
+
+    const registry = new ToolRegistry();
+    registry.register(echoTool);
+    const hooks = new HookEngine({ confirm: [], log: [], silent: [] });
+    const executor = new ToolExecutor(registry, hooks);
+
+    const agent = new NativeAgent({
+      modelClient: mockClient,
+      systemPrompt: 'You are helpful.',
+      toolRegistry: registry,
+      toolExecutor: executor,
+      maxIterations: 10,
+    });
+
+    const response = await agent.process('search for news');
+    expect(response).toContain('Tool loop detected');
+    expect(response).toContain('same thing'); // includes the last tool result
+    // Should break after 3 consecutive identical calls, not 10
+    expect(mockClient.chat).toHaveBeenCalledTimes(3);
+  });
+
   it('works without tools (backward compatible)', async () => {
     const mockClient: ModelClient = {
       chat: vi.fn().mockResolvedValue({