Rewrite task thin wrappers (cat, count, groupby)

[dmartmillan]

Description
Reduce openvariant/tasks/cat.py, count.py, and groupby.py to thin wrappers over their _core counterparts. plugin.py stays unchanged.

Files

• openvariant/tasks/cat.py — REWRITE
• openvariant/tasks/count.py — REWRITE
• openvariant/tasks/groupby.py — REWRITE
• openvariant/tasks/plugin.py — KEEP (pure Python, no changes)

Definition of Done

• All three functions have the same signatures as before
• Plugin callback in groupby still works via the Python plugin.py runner

Documentation needed: No external API changes.

Tests needed:

• Existing tests/test_tasks_cat.py, test_tasks_count.py, test_tasks_groupby.py must pass without modification