Decide the open v2 questions: audience, blocking, propose, batch limits #312

The open decisions listed at the end of #309, decided and written into #309 before the write tools are built.

1. Audience (me, project, organization:<id>) replacing scope, visibility and organizationId. Includes whether per-memory public disappears in favour of the project's visibility, and a migration rule for existing public memories in projects that are not public, and for upstream recall, which today takes only public memories from a fork's upstream.
2. Blocking a task: a status set with item_store, or an item_action with a reason.
3. propose: per call, as a setting on the agent key in the web app, or both.
4. Batch limits: item count, total size, mixed kinds in one batch, references between pages and tasks in one batch, and how a batch that partly needs review is proposed.
5. base token: format (opaque string), what it encodes per kind, and the error when it is stale.

Acceptance

• Each decision with its reason on #309; data migrations named as work in the matching subtask.