feat: add pqcat helper to inspect parquet files

A standalone tools/pqcat.py (head/tail/columns/info) wired as `cli.py pqcat`
and runnable directly via a PATH symlink, plus README docs.

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

cli.py

@@ -14,6 +14,7 @@ import click
 from pipeline.data.cli import data
 from pipeline.alpha.cli import alpha
 from pipeline.combo.cli import combo
+from tools.pqcat import pqcat
 
 
 @click.group()
@@ -37,6 +38,7 @@ def cli(log_level):
 cli.add_command(data)
 cli.add_command(alpha)
 cli.add_command(combo)
+cli.add_command(pqcat)
 
 
 if __name__ == "__main__":