feat(02-02): add DuckDB loader and CLI evidence command for gnomAD

- load_to_duckdb: Saves constraint DataFrame to gnomad_constraint table with provenance tracking
- query_constrained_genes: Queries constrained genes by LOEUF threshold (validates GCON-03 interpretation)
- evidence_cmd.py: CLI command group with gnomad subcommand (fetch->transform->load orchestration)
- Checkpoint-restart: Skips processing if gnomad_constraint table exists (--force to override)
- Full CLI: usher-pipeline evidence gnomad [--force] [--url URL] [--min-depth N] [--min-cds-pct N]

src/usher_pipeline/cli/main.py

@@ -11,6 +11,7 @@ import click
 from usher_pipeline import __version__
 from usher_pipeline.config.loader import load_config
 from usher_pipeline.cli.setup_cmd import setup
+from usher_pipeline.cli.evidence_cmd import evidence
 
 
 # Configure logging
@@ -95,8 +96,9 @@ def info(ctx):
         ctx.exit(1)
 
 
-# Register setup command
+# Register commands
 cli.add_command(setup)
+cli.add_command(evidence)
 
 
 if __name__ == '__main__':