fix(vcs): track mtDNA BED annotations and validate_haplogroup_f6.sh

The .gitignore filters by extension (*.bed, *.sh), which hid two classes of file
that are not data: the MITOMAP-derived mtDNA region annotations used by the F4
stratification, and validate_haplogroup_f6.sh, which is pipeline source code.
Added with `git add -f`.

Samplesheets are deliberately NOT included. docs/pipeline_notes.md#nextflow states
that the samplesheet holds participant clinical data and must stay out of version
control; the *.csv ignore rule is intentional, not an oversight. Sample-to-group
assignment belongs in the private study compendium, not in this shared pipeline
repository.

Also ignores *.bak_* so that samplesheet.csv.bak_3group, which carries the same
clinical columns but does not end in .csv, cannot be added by a careless `git add -A`.