Components.AI: Define follow-up queue ownership and attachment semantics

### Is your feature request related to a problem? Please describe the problem.

Queue ownership, queued-message editability, attachment ownership, and discard behavior are unresolved and affect both public API and resource lifetime.

### Describe the solution you'd like

Decide and document:

- Whether queue state belongs to `AgentContext` or a composer-owned service.
- Whether queued messages can be edited or only removed.
- Whether each queued message owns its attachments.
- How Stop disposes or preserves queued attachment resources.

Produce an API-ready design for implementation.

### Additional context

Leaf task for #68227 open design decisions 7-8.

Start by reading the parent issue #68227 and its design decisions 7–8, then review the Components.AI queue and attachment requirements described here. Produce an API-ready design that resolves queue ownership, queued-message editing or removal, attachment ownership, and Stop disposal or preservation behavior.

Written by the indexing model from the issue text.