refactor: split CLAUDE.md into focused documentation structure

- Restructure docs into topic-specific files to minimize context
- Create docs/workflow.md (Git + Agent workflow)
- Create docs/commands.md (Common failures + build commands)
- Create docs/architecture.md (System design + data flow)
- Create docs/testing.md (Test structure + guidelines)
- Rewrite CLAUDE.md as concise hub with links to detailed docs
- Update .gitignore to exclude data/ directory

Benefits:
- Reduced context size for AI assistants
- Faster reference lookups
- Better maintainability
- Topic-focused documentation

Closes #13

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

docs/testing.md

@@ -0,0 +1,213 @@
+# Testing Guidelines
+
+## Test Structure
+
+**54 tests** across four files. `asyncio_mode = "auto"` in pyproject.toml — async tests need no special decorator.
+
+The `settings` fixture in `conftest.py` provides safe defaults with test credentials and in-memory DB.
+
+### Test Files
+
+#### `tests/test_risk.py` (11 tests)
+- Circuit breaker boundaries
+- Fat-finger edge cases
+- P&L calculation edge cases
+- Order validation logic
+
+**Example:**
+```python
+def test_circuit_breaker_exact_threshold(risk_manager):
+    """Circuit breaker should trip at exactly -3.0%."""
+    with pytest.raises(CircuitBreakerTripped):
+        risk_manager.validate_order(
+            current_pnl_pct=-3.0,
+            order_amount=1000,
+            total_cash=10000
+        )
+```
+
+#### `tests/test_broker.py` (6 tests)
+- OAuth token lifecycle
+- Rate limiting enforcement
+- Hash key generation
+- Network error handling
+- SSL context configuration
+
+**Example:**
+```python
+async def test_rate_limiter(broker):
+    """Rate limiter should delay requests to stay under 10 RPS."""
+    start = time.monotonic()
+    for _ in range(15):  # 15 requests
+        await broker._rate_limiter.acquire()
+    elapsed = time.monotonic() - start
+    assert elapsed >= 1.0  # Should take at least 1 second
+```
+
+#### `tests/test_brain.py` (18 tests)
+- Valid JSON parsing
+- Markdown-wrapped JSON handling
+- Malformed JSON fallback
+- Missing fields handling
+- Invalid action validation
+- Confidence threshold enforcement
+- Empty response handling
+- Prompt construction for different markets
+
+**Example:**
+```python
+async def test_confidence_below_threshold_forces_hold(brain):
+    """Decisions below confidence threshold should force HOLD."""
+    decision = brain.parse_response('{"action":"BUY","confidence":70,"rationale":"test"}')
+    assert decision.action == "HOLD"
+    assert decision.confidence == 70
+```
+
+#### `tests/test_market_schedule.py` (19 tests)
+- Market open/close logic
+- Timezone handling (UTC, Asia/Seoul, America/New_York, etc.)
+- DST (Daylight Saving Time) transitions
+- Weekend handling
+- Lunch break logic
+- Multiple market filtering
+- Next market open calculation
+
+**Example:**
+```python
+def test_is_market_open_during_trading_hours():
+    """Market should be open during regular trading hours."""
+    # KRX: 9:00-15:30 KST, no lunch break
+    market = MARKETS["KR"]
+    trading_time = datetime(2026, 2, 3, 10, 0, tzinfo=ZoneInfo("Asia/Seoul"))  # Monday 10:00
+    assert is_market_open(market, trading_time) is True
+```
+
+## Coverage Requirements
+
+**Minimum coverage: 80%**
+
+Check coverage:
+```bash
+pytest -v --cov=src --cov-report=term-missing
+```
+
+Expected output:
+```
+Name                          Stmts   Miss  Cover   Missing
+-----------------------------------------------------------
+src/brain/gemini_client.py       85      5    94%   165-169
+src/broker/kis_api.py           120     12    90%   ...
+src/core/risk_manager.py         35      2    94%   ...
+src/db.py                        25      1    96%   ...
+src/main.py                     150     80    47%   (excluded from CI)
+src/markets/schedule.py          95      3    97%   ...
+-----------------------------------------------------------
+TOTAL                           510     103   80%
+```
+
+**Note:** `main.py` has lower coverage as it contains the main loop which is tested via integration/manual testing.
+
+## Test Configuration
+
+### `pyproject.toml`
+```toml
+[tool.pytest.ini_options]
+asyncio_mode = "auto"
+testpaths = ["tests"]
+python_files = ["test_*.py"]
+```
+
+### `tests/conftest.py`
+```python
+@pytest.fixture
+def settings() -> Settings:
+    """Provide test settings with safe defaults."""
+    return Settings(
+        KIS_APP_KEY="test_key",
+        KIS_APP_SECRET="test_secret",
+        KIS_ACCOUNT_NO="12345678-01",
+        GEMINI_API_KEY="test_gemini_key",
+        MODE="paper",
+        DB_PATH=":memory:",  # In-memory SQLite
+        CONFIDENCE_THRESHOLD=80,
+        ENABLED_MARKETS="KR",
+    )
+```
+
+## Writing New Tests
+
+### Naming Convention
+- Test files: `test_<module>.py`
+- Test functions: `test_<feature>_<scenario>()`
+- Use descriptive names that explain what is being tested
+
+### Good Test Example
+```python
+async def test_send_order_with_market_price(broker, settings):
+    """Market orders should use price=0 and ORD_DVSN='01'."""
+    # Arrange
+    stock_code = "005930"
+    order_type = "BUY"
+    quantity = 10
+
+    # Act
+    with patch.object(broker._session, 'post') as mock_post:
+        mock_post.return_value.__aenter__.return_value.status = 200
+        mock_post.return_value.__aenter__.return_value.json = AsyncMock(
+            return_value={"rt_cd": "0", "msg1": "OK"}
+        )
+
+        await broker.send_order(stock_code, order_type, quantity, price=0)
+
+    # Assert
+    call_args = mock_post.call_args
+    body = call_args.kwargs['json']
+    assert body['ORD_DVSN'] == '01'  # Market order
+    assert body['ORD_UNPR'] == '0'   # Price 0
+```
+
+### Test Checklist
+- [ ] Test passes in isolation (`pytest tests/test_foo.py::test_bar -v`)
+- [ ] Test has clear docstring explaining what it tests
+- [ ] Arrange-Act-Assert structure
+- [ ] Uses appropriate fixtures from conftest.py
+- [ ] Mocks external dependencies (API calls, network)
+- [ ] Tests edge cases and error conditions
+- [ ] Doesn't rely on test execution order
+
+## Running Tests
+
+```bash
+# All tests
+pytest -v
+
+# Specific file
+pytest tests/test_risk.py -v
+
+# Specific test
+pytest tests/test_brain.py::test_parse_valid_json -v
+
+# With coverage
+pytest -v --cov=src --cov-report=term-missing
+
+# Stop on first failure
+pytest -x
+
+# Verbose output with print statements
+pytest -v -s
+```
+
+## CI/CD Integration
+
+Tests run automatically on:
+- Every commit to feature branches
+- Every PR to main
+- Scheduled daily runs
+
+**Blocking conditions:**
+- Test failures → PR blocked
+- Coverage < 80% → PR blocked (warning only for main.py)
+
+**Non-blocking:**
+- `mypy --strict` errors (type hints encouraged but not enforced)
+- `ruff check` warnings (must be acknowledged)