Create sub-groups within a group

"As a group admin, I want to create sub-groups within my group, so members can organize discussions by topic."

Besides linking related groups (see https://github.com/swicg/groups/issues/31), we implemented a tree structure where:

• Groups can contain child groups (channels/topics)
• Channels inherit properties from parent groups
• Members can browse by channel or see all group content

Existing approaches:

• Schema.org: hasPart/isPartOf for parent-child relationships
• Nodebb: Nested categories

Suggested representation for a parent group:

{
  "type": "Group",
  "id": "https://example.org/groups/programming",
  "name": "Programming",
  "attributedTo": ["https://example.org/actors/alice"],
  "hasPart": [
    {"type": "Group", "id": "https://example.org/groups/programming/rust", "name": "Rust"},
    {"type": "Group", "id": "https://example.org/groups/programming/python", "name": "Python"},
    {"type": "Group", "id": "https://example.org/groups/programming/elixir", "name": "Elixir"}
  ]

Suggested representation for a child group:

{
  "type": "Group",
  "id": "https://example.org/groups/programming/rust",
  "name": "Rust",
  "isPartOf": "https://example.org/groups/programming",
  "attributedTo": ["https://example.org/groups/programming/attributedTo"],
  "interactionPolicy": {
    "canJoin": { "automaticApproval": ["https://example.org/groups/programming/followers"] },
    "canModerate": { "automaticApproval": ["https://example.org/groups/programming/attributedTo"] }

Key patterns:

• isPartOf links to parent group (we could also use context but this makes more sense if also using hasPart for child groups)
• Child can reference parent's collections in interactionPolicy for permission inheritance
• Child can have its own attributedTo or reference parent's

Start with issue #32 and the linked groups issue #31, then compare the proposed parent and child Group representations with the existing Groups Task Force conventions. Review the hasPart/isPartOf relationships, inherited properties, and interactionPolicy examples; done requires an agreed representation for nested groups and permission inheritance.