[Subtask] Design doc for tag-based access control

### Describe the subtask

Write the design for expressing access rules as tag-bound policies: a `Policy` of type `system_access_control` whose `content` carries an action, a role condition and a scope, bound to a tag.

Covers the `AccessControlContent` schema and its `validate()`, evaluation and cache freshness, composition with RBAC, the administration surface, and lifecycle. Four questions are left open with their options rather than decided: where tags are evaluated, composition when a tag allows and RBAC denies, what happens when a referenced role is deleted, and whether object-scoped `APPLY_TAG` ships in this version.

Discussion: #12619

### Parent issue

#12758

Start by reading discussion #12619 and the parent issue #12758. Define the AccessControlContent schema, validation, evaluation and cache freshness, RBAC composition, administration, and lifecycle, then document decisions for the four open questions and the completed design scope.

Written by the indexing model from the issue text.