Add one level of submemories (bags) #295

See #290, section 3.

Work

• Optional parent memory in the same project and scope. A memory with active children is a bag; there is no bag flag.
• Rules enforced in the service: exactly one level, no critical children, no cross-project or cross-scope parents, no personal memory under a project bag.
• memory_store sets and clears the parent; item_get on a bag returns visible children with title and summary using the continuation contract from #219.
• item_search finds children directly and returns parent.
• Archiving a bag moves its children to the top level.
• Hygiene score: stale bag (a child changed after the last bag revision that changed its summary, weighted by child weight) and lopsided bag (one child or more than the maximum).
• Web: set parent in the editor, show children on the bag's detail page and the bag on each child.

Acceptance

• Counts, titles and continuation metadata include only children the reader may see; a public bag with private children reveals nothing about them.
• Tests for every rule above, for archive and for concurrent parent changes.
• A save that leaves the bag summary unchanged does not clear a stale-bag finding.