[berntpopp]

Summary

• Add optional gatk_resources.extra_panel_of_normals to config, schema, and docs (README.md, config/README.md).
• Add pon_combine rule in workflow/rules/mutect2.smk to align header contigs with the reference dictionary, drop records on contigs absent from the reference, and merge cohort pon.vcf.gz with extra_panel_of_normals (bcftools concat -a -D) into pon.combined.vcf.gz.
• Update pon_mask_hotspots and get_pon() in workflow/rules/common.smk so combined PoNs are masked into pon.combined.masked.vcf.gz (or pon.combined.vcf.gz when unmasked), leaving prebuilt panel_of_normals behaviour unchanged.
• Add unit, bcftools shell integration, schema, and Snakemake dry-run tests covering prebuilt, cohort, and cohort+extra PoNs (each with and without hotspot masking).

Closes #28