feat(copy): add path_groups for destination-scoped path sets

[EspenAlbert]

Summary

• Add path_groups on SrcConfig and include_groups on Destination so files can be scoped to a subset of destinations without duplicating skip_file_patterns
• A destination's effective paths = config.paths + paths from its include_groups
• Pydantic model_validator rejects unknown and duplicate group references at parse time

Config example

path_groups:
  regions:
    - src_path: .github/dev/.gitignore
    - src_path: .github/dev/extract_regions.py

destinations:
  - name: azure
    include_groups: [regions]
  - name: aws
    # receives only config.paths

Changes

• models.py: add path_groups field to SrcConfig, include_groups field to Destination, resolve_paths() method, model validator (unknown + duplicate group rejection)
• cmd_copy.py: replace config.paths with config.resolve_paths(dest) in _sync_paths
• models_test.py: unit tests for resolve_paths (no groups, single group, multiple groups) and validation (unknown group, duplicate group)