feat: implement daily trading mode with batch decisions (issue #57)

Add API-efficient daily trading mode for Gemini Free tier compatibility:

## Features

- **Batch Decisions**: GeminiClient.decide_batch() analyzes multiple stocks
  in a single API call using compressed JSON format
- **Daily Trading Mode**: run_daily_session() executes N sessions per day
  at configurable intervals (default: 4 sessions, 6 hours apart)
- **Mode Selection**: TRADE_MODE env var switches between daily (batch)
  and realtime (per-stock) modes
- **Requirements Log**: docs/requirements-log.md tracks user feedback
  chronologically for project evolution

## Configuration

- TRADE_MODE: "daily" (default) | "realtime"
- DAILY_SESSIONS: 1-10 (default: 4)
- SESSION_INTERVAL_HOURS: 1-24 (default: 6)

## API Efficiency

- 2 markets × 4 sessions = 8 API calls/day (within Free tier 20 calls)
- 3 markets × 4 sessions = 12 API calls/day (within Free tier 20 calls)

## Testing

- 9 new batch decision tests (all passing)
- All existing tests maintained (298 passed)

## Documentation

- docs/architecture.md: Trading Modes section with daily vs realtime
- CLAUDE.md: Requirements Management section
- docs/requirements-log.md: Initial entries for API efficiency needs

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

tests/test_brain.py

@@ -152,3 +152,121 @@ class TestPromptConstruction:
         assert "JSON" in prompt
         assert "action" in prompt
         assert "confidence" in prompt
+
+
+# ---------------------------------------------------------------------------
+# Batch Decision Making
+# ---------------------------------------------------------------------------
+
+
+class TestBatchDecisionParsing:
+    """Batch response parser must handle JSON arrays correctly."""
+
+    def test_parse_valid_batch_response(self, settings):
+        client = GeminiClient(settings)
+        stocks_data = [
+            {"stock_code": "AAPL", "current_price": 185.5},
+            {"stock_code": "MSFT", "current_price": 420.0},
+        ]
+        raw = """[
+            {"code": "AAPL", "action": "BUY", "confidence": 85, "rationale": "Strong momentum"},
+            {"code": "MSFT", "action": "HOLD", "confidence": 50, "rationale": "Wait for earnings"}
+        ]"""
+
+        decisions = client._parse_batch_response(raw, stocks_data, token_count=100)
+
+        assert len(decisions) == 2
+        assert decisions["AAPL"].action == "BUY"
+        assert decisions["AAPL"].confidence == 85
+        assert decisions["MSFT"].action == "HOLD"
+        assert decisions["MSFT"].confidence == 50
+
+    def test_parse_batch_with_markdown_wrapper(self, settings):
+        client = GeminiClient(settings)
+        stocks_data = [{"stock_code": "AAPL", "current_price": 185.5}]
+        raw = """```json
+[{"code": "AAPL", "action": "BUY", "confidence": 90, "rationale": "Good"}]
+```"""
+
+        decisions = client._parse_batch_response(raw, stocks_data, token_count=100)
+
+        assert decisions["AAPL"].action == "BUY"
+        assert decisions["AAPL"].confidence == 90
+
+    def test_parse_batch_empty_response_returns_hold_for_all(self, settings):
+        client = GeminiClient(settings)
+        stocks_data = [
+            {"stock_code": "AAPL", "current_price": 185.5},
+            {"stock_code": "MSFT", "current_price": 420.0},
+        ]
+
+        decisions = client._parse_batch_response("", stocks_data, token_count=100)
+
+        assert len(decisions) == 2
+        assert decisions["AAPL"].action == "HOLD"
+        assert decisions["AAPL"].confidence == 0
+        assert decisions["MSFT"].action == "HOLD"
+
+    def test_parse_batch_malformed_json_returns_hold_for_all(self, settings):
+        client = GeminiClient(settings)
+        stocks_data = [{"stock_code": "AAPL", "current_price": 185.5}]
+        raw = "This is not JSON"
+
+        decisions = client._parse_batch_response(raw, stocks_data, token_count=100)
+
+        assert decisions["AAPL"].action == "HOLD"
+        assert decisions["AAPL"].confidence == 0
+
+    def test_parse_batch_not_array_returns_hold_for_all(self, settings):
+        client = GeminiClient(settings)
+        stocks_data = [{"stock_code": "AAPL", "current_price": 185.5}]
+        raw = '{"code": "AAPL", "action": "BUY", "confidence": 90, "rationale": "Good"}'
+
+        decisions = client._parse_batch_response(raw, stocks_data, token_count=100)
+
+        assert decisions["AAPL"].action == "HOLD"
+        assert decisions["AAPL"].confidence == 0
+
+    def test_parse_batch_missing_stock_gets_hold(self, settings):
+        client = GeminiClient(settings)
+        stocks_data = [
+            {"stock_code": "AAPL", "current_price": 185.5},
+            {"stock_code": "MSFT", "current_price": 420.0},
+        ]
+        # Response only has AAPL, MSFT is missing
+        raw = '[{"code": "AAPL", "action": "BUY", "confidence": 85, "rationale": "Good"}]'
+
+        decisions = client._parse_batch_response(raw, stocks_data, token_count=100)
+
+        assert decisions["AAPL"].action == "BUY"
+        assert decisions["MSFT"].action == "HOLD"
+        assert decisions["MSFT"].confidence == 0
+
+    def test_parse_batch_invalid_action_becomes_hold(self, settings):
+        client = GeminiClient(settings)
+        stocks_data = [{"stock_code": "AAPL", "current_price": 185.5}]
+        raw = '[{"code": "AAPL", "action": "YOLO", "confidence": 90, "rationale": "Moon"}]'
+
+        decisions = client._parse_batch_response(raw, stocks_data, token_count=100)
+
+        assert decisions["AAPL"].action == "HOLD"
+
+    def test_parse_batch_low_confidence_becomes_hold(self, settings):
+        client = GeminiClient(settings)
+        stocks_data = [{"stock_code": "AAPL", "current_price": 185.5}]
+        raw = '[{"code": "AAPL", "action": "BUY", "confidence": 65, "rationale": "Weak"}]'
+
+        decisions = client._parse_batch_response(raw, stocks_data, token_count=100)
+
+        assert decisions["AAPL"].action == "HOLD"
+        assert decisions["AAPL"].confidence == 65
+
+    def test_parse_batch_missing_fields_gets_hold(self, settings):
+        client = GeminiClient(settings)
+        stocks_data = [{"stock_code": "AAPL", "current_price": 185.5}]
+        raw = '[{"code": "AAPL", "action": "BUY"}]'  # Missing confidence and rationale
+
+        decisions = client._parse_batch_response(raw, stocks_data, token_count=100)
+
+        assert decisions["AAPL"].action == "HOLD"
+        assert decisions["AAPL"].confidence == 0