Support multiple Unix groups via --groups

[Copilot]

Replace the single-group --group parameter with --groups, accepting one or more group names. Member sets are unioned across all specified groups.

Changes

• CLI: --group → --groups with nargs='+'
• main(): reads groups: list from config; builds union of members across all groups
• get_directory_stats(): group: str | None → groups: list[str] | None; collects members from all groups before filtering
• _build_directory_entry(): stores "groups" (list) in output dict instead of scalar "group"
• report_directory(): prints Groups : group1, group2
• config.yaml: group: name → groups:\n - name (list format)
• Tests: all call sites updated; 3 new multi-group tests added

Usage

# Single group (unchanged behavior)
python3 ontrack.py --config config.yaml --groups researchers

# Multiple groups
python3 ontrack.py --config config.yaml --groups researchers analysts

# config.yaml
groups:
  - researchers
  - analysts