test: Add extensive coverage for discussion metrics and compression

- Add tests/test_discussion_compression.py to verify AI sub-agent compression logic across Gemini, Anthropic, DeepSeek, and Gemini CLI providers.
- Add tests/test_discussion_metrics.py to verify AppController correctly extracts and accumulates token usage (input/output/cache) and logs token history.

tests/test_discussion_metrics.py

@@ -0,0 +1,63 @@
+import pytest
+from src.app_controller import AppController
+
+@pytest.fixture
+def controller():
+    c = AppController()
+    c.init_state()
+    return c
+
+def test_on_comms_entry_updates_metrics(controller: AppController):
+    # Initial state
+    assert controller.session_usage["input_tokens"] == 0
+    assert controller.session_usage["output_tokens"] == 0
+    assert len(controller._token_history) == 0
+
+    # Simulate an AI response with usage
+    payload = {
+        "text": "Hello world",
+        "usage": {
+            "input_tokens": 100,
+            "output_tokens": 50,
+            "cache_read_input_tokens": 10,
+            "cache_creation_input_tokens": 5,
+            "total_tokens": 165
+        },
+        "model": "test-model"
+    }
+    entry = {
+        "kind": "response",
+        "payload": payload,
+        "ts": "2024-01-01T00:00:00"
+    }
+
+    controller._on_comms_entry(entry)
+
+    # Verify metrics updated
+    assert controller.session_usage["input_tokens"] == 100
+    assert controller.session_usage["output_tokens"] == 50
+    assert controller.session_usage["cache_read_input_tokens"] == 10
+    assert controller.session_usage["cache_creation_input_tokens"] == 5
+    assert controller.session_usage["total_tokens"] == 165
+
+    # Verify token history recorded
+    assert len(controller._token_history) == 1
+    assert controller._token_history[0]["input"] == 100
+    assert controller._token_history[0]["output"] == 50
+    assert controller._token_history[0]["model"] == "test-model"
+
+def test_on_comms_entry_accumulates_metrics(controller: AppController):
+    # First response
+    controller._on_comms_entry({
+        "kind": "response",
+        "payload": {"usage": {"input_tokens": 100, "output_tokens": 50}}
+    })
+    # Second response
+    controller._on_comms_entry({
+        "kind": "response",
+        "payload": {"usage": {"input_tokens": 200, "output_tokens": 20}}
+    })
+
+    assert controller.session_usage["input_tokens"] == 300
+    assert controller.session_usage["output_tokens"] == 70
+    assert len(controller._token_history) == 2