Rule book: rules as versioned content published through change sets #2634

Part of #2419 (F-RUL). The first slice: the rule aggregate, stored as versioned content and published through change sets (RUL-01, RUL-02, RUL-03). It builds on the change set writes of #2520 (SCH-05).

Scope

• Core (FluentCMS.Core.Rules):
  • A rule has a slug and a definition: its kind, prose text, scope and severity.
    • Kinds (RUL-02): voice and style, factual, structural, and permission and routing.
    • The scope (RUL-03) names content types and fields, and keeps a locale slot that nothing reads in M1.
    • Severity is block or warn.
  • A rule's status follows its versions (RUL-03):
    • draft until a version is published
    • active while a published version is live
    • retired once a change set retires it
  • IRuleDirectory:
    • read the list, the latest version, the active version, and any earlier version
    • create, revise and retire, each into a ChangeSetTarget
  • Each write stores a version and records it as an operation of its change set, with their audit events in one unit of work. A write never changes what is active; only publishing its change set does.
  • Only a person writes rules (M1: any administrator); an agent and an engine are refused. Agents read the rule book in a later slice.
• Change sets: publish, revert and the diff handle rule items, as they handle content types.
• Infrastructure: tables for rules and their versions, with migrations for SQLite and PostgreSQL.
• CONTRIBUTING.md: rules.

Out of scope

• Proposing checks from a rule (RUL-04), enforced and advisory labelling and the agent tools (RUL-05, RUL-06), the instruction check on rule text (RUL-07), and proposing a rule from the desk (RUL-08): the epic's next slices.
• Evaluating permission and routing rules (F-RTE), locale scopes in use (I18-02), and a rule author role (USR-02).

Acceptance

• A person writes "never claim best in class" as a voice and style rule for product pages into a change set. It is a draft at version 1 until the set is published, then active.
• A revision stores version 2 and leaves version 1 active until its set is published.
• Retiring a rule through a change set retires it when the set is published, and reverting that set makes it active again.
• The change set's diff shows a rule's text and scope before and after.
• A rule without text, with an unknown kind or severity, or with a slug already taken, is refused with what to change, and nothing is stored.
• An agent and an engine are refused every rule write.
• Everything above passes on SQLite and PostgreSQL, with migrations for both and a working Down.