Migrate sentry-resque to AJ-first integration #2936

Part of #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)