feat: unified logging module and remove redundant first_run logic

- Add logger.py with TimedRotatingFileHandler (daily rotation, 30-day retention)
- Replace all print() in migrate.py with logging calls
- Add logging to sync.py for sync status tracking
- Refactor watch.py to use logger.py, remove duplicate first_run branch
- All logs now go to logs/app.log consistently
- Add logging config section to config.yaml, add logs/ to .gitignore

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

logger.py

@@ -0,0 +1,65 @@
+"""
+Unified logging module.
+Provides a pre-configured logger with console + rotating file handlers.
+"""
+
+import logging
+import os
+from logging.handlers import TimedRotatingFileHandler
+
+
+def setup_logger(name='app', log_dir=None, level='INFO', max_keep_days=30):
+    """Create and configure a logger.
+
+    Args:
+        name: Logger name (used as basename for log file).
+        log_dir: Directory for log files. Defaults to <project_root>/logs.
+        level: Logging level string (DEBUG, INFO, WARNING, ERROR).
+        max_keep_days: Number of days to retain rotated log files.
+
+    Returns:
+        Configured logging.Logger instance.
+    """
+    if log_dir is None:
+        log_dir = 'logs'
+    # Resolve relative paths against project root
+    project_root = os.path.dirname(os.path.abspath(__file__))
+    if not os.path.isabs(log_dir):
+        log_dir = os.path.join(project_root, log_dir)
+    os.makedirs(log_dir, exist_ok=True)
+
+    logger = logging.getLogger(name)
+    logger.setLevel(getattr(logging, level.upper(), logging.INFO))
+
+    # Avoid adding duplicate handlers on repeated calls
+    if logger.handlers:
+        return logger
+
+    fmt = logging.Formatter(
+        '%(asctime)s [%(levelname)s] %(message)s',
+        datefmt='%Y-%m-%d %H:%M:%S'
+    )
+
+    # Console handler
+    ch = logging.StreamHandler()
+    ch.setLevel(getattr(logging, level.upper(), logging.INFO))
+    ch.setFormatter(fmt)
+    logger.addHandler(ch)
+
+    # File handler — rotate daily, keep max_keep_days backups
+    log_file = os.path.join(log_dir, f'{name}.log')
+    fh = TimedRotatingFileHandler(
+        log_file, when='midnight', interval=1,
+        backupCount=max_keep_days, encoding='utf-8'
+    )
+    fh.setLevel(getattr(logging, level.upper(), logging.INFO))
+    fh.setFormatter(fmt)
+    fh.suffix = '%Y-%m-%d'
+    logger.addHandler(fh)
+
+    return logger
+
+
+def get_logger(name='app'):
+    """Get an existing logger by name (does not reconfigure)."""
+    return logging.getLogger(name)