Migrate `sentry-resque` to AJ-first integration

Part of getsentry/sentry-ruby#2932.

Same shape as the `sentry-delayed_job` migration; slightly more surface but structurally identical treatment. The raw-Resque code path is preserved unchanged.

## Tasks

* **3.1** Unregister from `skippable_job_adapters` (per Phase 1.6 decision). Strip the Railtie block at `sentry-resque.rb#L17-L23`.
* **3.2** Add a thin spec file in `sentry-resque` that wires the AJ Resque adapter into the shared contract via `it_behaves_like "a Sentry-instrumented ActiveJob backend"`. Retain the existing raw-Resque specs unchanged.
* **3.3** Reposition README + gemspec as maintenance-mode: raw Resque path preserved; point AJ users at `sentry-rails`.

## Depends on

Phase 1 (ActiveJob core integration in `sentry-rails`)