#444 Docs: update datastore reference for lazy hydration, SyncContext, and SyncCapabilities

What needs updating

The datastore efficiency overhaul shipped several new interfaces and features that need documentation in the swamp-club reference docs.

1. Datastore configuration reference

Document the new hydrationStrategy field on datastore config:

```yaml datastore: type: "@swamp/s3-datastore" hydrationStrategy: lazy # or "full" (default) config: bucket: my-bucket region: us-east-1 ```

And the CLI flag: `swamp datastore setup extension @swamp/s3-datastore --hydration-strategy lazy --config '...'`

2. DatastoreSyncService interface reference

New methods and fields added across Phase 1-3:

• `capabilities?(): SyncCapabilities` — advertise scopedSync, lazyHydration
• `hydrateFile?(relPath, options?): Promise` — on-demand single file download
• `context?: SyncContext` on `DatastoreSyncOptions` — domain context with models array
• `metadataOnly?: boolean` on `DatastoreSyncOptions` — metadata-only pull filter
• `SyncContext` interface — `{ models?: ReadonlyArray<{ modelType, modelId }> }`
• `SyncCapabilities` interface — `{ scopedSync?, lazyHydration? }`

3. Extension author guide

How to implement scoped sync and lazy hydration in a custom datastore extension:

• Implement `capabilities()` returning `{ scopedSync: true, lazyHydration: true }`
• Handle `options?.context?.models` in `pullChanged`/`pushChanged` for scoped sync
• Handle `options?.metadataOnly` in `pullChanged` for lazy setup
• Implement `hydrateFile` for on-demand content download
• Per-path dirty tracking via `markDirty(options?.relPath)`

4. Conformance test reference

Document `assertSyncServiceConformance()` from `@systeminit/swamp-testing`.

Reference implementations

• `@swamp/s3-datastore` and `@swamp/gcs-datastore` have all features implemented
• `design/datastores.md` in the swamp repo has the technical design doc