[Feature] Unify persistence plugin registration (self-activating, top-level url, uniform handles, SQLite plugin, ConfigKeys)

[pathosDev]

The four registerXxxPlugins entry points have divergent shapes and none self-activate — users must both register in code and set the HOCON plugin key (footgun #377). DX unification:

• activate (builder withActivate, default true) on the Register*PluginsOptions composites: register + set the active journal/snapshot in one call; an explicit HOCON key still wins.
• Top-level withUrl/withPoolConfig on the composite (merged onto every leaf like the existing shared withPool), so the URL is written once.
• Uniform return PersistenceHandles { journal?, snapshotStore?, durableStateStore?, close() } for all four.
• New registerSqlitePlugins + plugin id (SQLite is the only backend with no registration story) with its own XOptions family.
• Complete src/config/ConfigKeys.ts with the postgres/mariadb/sqlite plugin ids + durable-state keys; have PersistenceExtension read via ConfigKeys instead of string literals.
• Add a Postgres example and an end-to-end register+config integration test (currently none).

Depends on #377. Part of Phase 3.