Add slim caller prompt builder (~400 tokens)

tests/test_caller_prompt_slim.py

@@ -0,0 +1,23 @@
+from backend.main import get_caller_prompt_slim
+
+
+def test_slim_prompt_includes_identity_and_situation():
+    caller = {
+        "name": "Danny",
+        "identity": "A plumber who inherited a taxidermy shop",
+        "situation": "Getting strange calls about taxidermy",
+        "reason_calling": "Someone left a note",
+        "secret_want": "Permission to throw it all away",
+        "specific_details": ["elk head in basement", "note said she forgot"],
+    }
+    prompt = get_caller_prompt_slim(caller)
+    assert "Danny" in prompt
+    assert "taxidermy shop" in prompt
+    assert "strange calls" in prompt
+    assert "elk head" in prompt
+    assert "she forgot" in prompt
+    assert "Permission to throw it all away" in prompt
+    assert "React to what Luke says" in prompt
+    assert "Stay in character" in prompt
+    # Assert it's under 800 tokens (roughly 3200 chars) — should be ~400 tokens
+    assert len(prompt) < 3200