refactor: split ZammadClient into three concerns (ClientFactory, ImpersonationHandler, slim client)

Split the monolithic ZammadClient into focused classes:

- ClientFactory → GuzzleClientFactory implements ClientFactoryInterface
  Guzzle wiring lives exclusively in GuzzleClientFactory::buildClient()
  Non-Guzzle via new ZammadClient(new RequestHandler(...))

- ImpersonationHandler — stateless decorator implementing
  RequestHandlerInterface. Injects From header on every request
  including getRaw(). No shared mutable state.

- ZammadClient — slimmed from ~210 to ~79 lines. Only repo() and
  getHandler(). Repository access via typed, explicit, IDE-friendly
  methods (ticket(), user(), group(), etc.) implemented directly on
  the class.

Removals and cleanups:

- __call, aliasMap, resolveAlias — replaced by explicit typed methods
- RequestHandler:: — shared mutable state removed
- RequestHandlerInterface::setOnBehalfOfUser/getOnBehalfOfUser — removed
- RequestHandlerInterface::getRaw() extended with $headers parameter
  for ImpersonationHandler compatibility
- onBehalfOf() / performOnBehalfOf() — not a Client concern;
  use new ZammadClient(new ImpersonationHandler($handler, $userId))
- getListKey() default $this->resourcePath; 10 identical impls removed

Namespace structure:

  Core/Contracts/   — ClientInterface, ClientFactoryInterface
  Core/Repository/  — AbstractRepository, RepositoryRegistry, PaginatedList, Resource, ResponseParser, DtoHydrator
  Core/Transport/   — RequestHandler, RetryAfterMiddleware, ImpersonationHandler, HttpPageFetcher
  Core/Traits/      — RepositoryAccessors (opt-in for custom ClientInterface impls), HasTimestamps, HydratesFromArray, SerializesToArray
  Factory/          — GuzzleClientFactory

Bug fixes:

- HttpPageFetcher::extractIndexResults() now reads total_count from
  API response instead of hard-coding null

Author: derpixler

.gitignore

@@ -2,3 +2,5 @@
 /.phpunit.cache/
 /composer.lock
 /build/
+/.opencode/
+/playground.php