feat: Kysely adapter (@rafters/ledger/kysely)

[ssilvius]

Goal

Add a Kysely ORM adapter alongside the existing Drizzle adapter. Kysely is the ORM better-auth uses internally, making this a native integration path.

Blocked by: better-auth 1.6 upgrade evaluation (may change the integration surface)

Subpath Exports

Subpath	What
./kysely	Adapter barrel
./kysely/schema/sqlite	Audit log table definition (Kysely createTable)
./kysely/schema/pg	PostgreSQL variant
./kysely/schema/mysql	MySQL variant
./kysely/soft-delete/sqlite	Soft-delete column helpers
./kysely/soft-delete/pg	PG variant
./kysely/soft-delete/mysql	MySQL variant

Adapter Must Implement

• Query filters: notDeleted, onlyDeleted, includingDeleted (Kysely query builder)
• Automatic soft-delete: interceptor that converts delete to update
• Audit logging: insert audit entries via Kysely
• Audit query: getRecordHistory via Kysely
• GDPR purge: purgeUserData via Kysely query builder
• Schema definitions: audit log table per dialect

Design Constraints

• Core types, context, pure helpers come from @rafters/ledger (no duplication)
• Follow the same pattern as src/drizzle/ -- same file structure, same exports shape
• Kysely is an optional peer dependency
• Tests in test/kysely/ mirroring test/drizzle/

Out of Scope

• Prisma adapter (evaluate if demand exists)
• Changes to core
• better-auth plugin changes (evaluate after 1.6)

Complete When

All Drizzle adapter functionality has a Kysely equivalent. Tests pass. Docs updated with Kysely examples.