feat: add variant-info command to retrieve sample carriersFeat/variant info

[dlopez-bioinfo]

Summary

• Adds variant-info CLI command and Python API method to list all samples carrying a specific variant
• Each carrier includes: genotype (het/hom/alt), FILTER status (PASS/FAIL), sex, technology, and phenotype codes
• Supports the same sample filters as query (--phenotype, --sex, --tech)
• Output formats: text (aligned table), TSV, and JSON

Closes #18

Test plan

• pytest tests/test_variant_info.py -v — 26 new tests, all passing
• pytest --tb=short -q — full suite (470 tests), no regressions
• afquery variant-info --db demo_db/ --locus chr1:1500 --format text
• afquery variant-info --db demo_db/ --locus chr1:1500 --ref A --alt T --format json

[codecov-commenter]

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Codecov Report

❌ Patch coverage is 88.42975% with 14 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
src/afquery/query.py	81.35%	6 Missing and 5 partials ⚠️
src/afquery/variant_info.py	66.66%	2 Missing ⚠️
src/afquery/cli.py	97.43%	0 Missing and 1 partial ⚠️

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