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>

docs/architecture.md

@@ -2,7 +2,42 @@
 
 ## Overview
 
-Self-evolving AI trading agent for global stock markets via KIS (Korea Investment & Securities) API. The main loop in `src/main.py` orchestrates four components in a 60-second cycle per stock across multiple markets.
+Self-evolving AI trading agent for global stock markets via KIS (Korea Investment & Securities) API. The main loop in `src/main.py` orchestrates four components across multiple markets with two trading modes: daily (batch API calls) or realtime (per-stock decisions).
+
+## Trading Modes
+
+The system supports two trading frequency modes controlled by the `TRADE_MODE` environment variable:
+
+### Daily Mode (default)
+
+Optimized for Gemini Free tier API limits (20 calls/day):
+
+- **Batch decisions**: 1 API call per market per session
+- **Fixed schedule**: 4 sessions per day at 6-hour intervals (configurable)
+- **API efficiency**: Processes all stocks in a market simultaneously
+- **Use case**: Free tier users, cost-conscious deployments
+- **Configuration**:
+  ```bash
+  TRADE_MODE=daily
+  DAILY_SESSIONS=4        # Sessions per day (1-10)
+  SESSION_INTERVAL_HOURS=6  # Hours between sessions (1-24)
+  ```
+
+**Example**: With 2 markets (US, KR) and 4 sessions/day = 8 API calls/day (within 20 call limit)
+
+### Realtime Mode
+
+High-frequency trading with individual stock analysis:
+
+- **Per-stock decisions**: 1 API call per stock per cycle
+- **60-second interval**: Continuous monitoring
+- **Use case**: Production deployments with Gemini paid tier
+- **Configuration**:
+  ```bash
+  TRADE_MODE=realtime
+  ```
+
+**Note**: Realtime mode requires Gemini API subscription due to high call volume.
 
 ## Core Components
 
@@ -192,6 +227,11 @@ MAX_LOSS_PCT=3.0
 MAX_ORDER_PCT=30.0
 ENABLED_MARKETS=KR,US_NASDAQ  # Comma-separated market codes
 
+# Trading Mode (API efficiency)
+TRADE_MODE=daily              # daily | realtime
+DAILY_SESSIONS=4              # Sessions per day (daily mode only)
+SESSION_INTERVAL_HOURS=6      # Hours between sessions (daily mode only)
+
 # Telegram Notifications (optional)
 TELEGRAM_BOT_TOKEN=1234567890:ABCdefGHIjklMNOpqrsTUVwxyz
 TELEGRAM_CHAT_ID=123456789

docs/requirements-log.md

@@ -0,0 +1,28 @@
+# Requirements Log
+
+프로젝트 진화를 위한 사용자 요구사항 기록.
+
+이 문서는 시간순으로 사용자와의 대화에서 나온 요구사항과 피드백을 기록합니다.
+새로운 요구사항이 있으면 날짜와 함께 추가하세요.
+
+---
+
+## 2026-02-05
+
+### API 효율화
+- Gemini API는 귀중한 자원. 종목별 개별 호출 대신 배치 호출 필요
+- Free tier 한도(20 calls/day) 고려하여 일일 몇 차례 거래 모드로 전환
+- 배치 API 호출로 여러 종목을 한 번에 분석
+
+### 거래 모드
+- **Daily Mode**: 하루 4회 거래 세션 (6시간 간격) - Free tier 호환
+- **Realtime Mode**: 60초 간격 실시간 거래 - 유료 구독 필요
+- `TRADE_MODE` 환경변수로 모드 선택
+
+### 진화 시스템
+- 사용자 대화 내용을 문서로 기록하여 향후에도 의도 반영
+- 프롬프트 품질 검증은 별도 이슈로 다룰 예정
+
+### 문서화
+- 시스템 구조, 기능별 설명 등 코드 문서화 항상 신경쓸 것
+- 새로운 기능 추가 시 관련 문서 업데이트 필수