refactor: class-based alpha factory + month-partitioned data pipeline

Replace the old signal/strategy/backtest modules with a decoupled
data → alpha → combo pipeline (parquet between phases, .pq extension).

Alphas:
- BaseAlpha + @register_alpha factory/plugin registry; one file per
  built-in (reversal, reversal_vol, momentum); external alphas via
  --alpha-module. Alphas are z-scored position weights, not predictors.

Data:
- baostock primary / akshare fallback, treated consistently.
- New --universe all (~5000 A-shares via query_all_stock, filtered).
- login-once batch downloader; empty-string OHLCV coerced to NaN.
- Month-partitioned dataset {output_dir}/{universe}/month=YYYY-MM/*.pq
  with chunked durability flushes; --data-path is the dataset dir.

CLI logs at INFO by default (--log-level) so progress is visible.
Docs (README, CLAUDE.md) updated incl. pipeline diagram and roadmap
TODOs for portfolio construction / backtest / paper trading.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

pipeline/data/cli.py

@@ -0,0 +1,44 @@
+"""CLI for data download phase."""
+
+import click
+from datetime import date
+
+from pipeline.data.downloader import download_universe
+
+
+@click.group(name="data")
+def data():
+    """Download and manage market data."""
+
+
+@data.command("download")
+@click.option(
+    "--universe", default="csi500",
+    help="Which universe: hs300, csi500, all (~5000 A-shares), file path, or comma-separated symbols",
+)
+@click.option("--start-date", default="2017-01-01", help="Start date YYYY-MM-DD")
+@click.option("--end-date", default=str(date.today()), help="End date YYYY-MM-DD")
+@click.option("--output-dir", default="data/daily_bars", help="Root for the partitioned dataset")
+@click.option("--symbols", default=0, type=int, help="Max symbols (0=all)")
+@click.option("--chunk-size", default=300, type=int, help="Symbols per durability flush")
+@click.option("--adjust", default="qfq", help="Price adjust: qfq, hfq, or none")
+def download(universe, start_date, end_date, output_dir, symbols, chunk_size, adjust):
+    """Download daily bars into a month-partitioned parquet dataset.
+
+    Writes ``{output_dir}/{universe}/month=YYYY-MM/*.pq``. Point ``alpha
+    compute --data-path`` at that dataset directory.
+    """
+    stats = download_universe(
+        universe=universe,
+        start_date=start_date,
+        end_date=end_date,
+        output_dir=output_dir,
+        max_symbols=symbols,
+        chunk_size=chunk_size,
+        adjust=adjust,
+    )
+    click.echo(
+        f"\nSummary: {stats['n_symbols']}/{stats['n_requested']} symbols, "
+        f"{stats['n_rows']:,} bars, {stats['date_min']} → {stats['date_max']}"
+    )
+    click.echo(f"Dataset: {stats['dataset_path']}")