refactor(rules): split angular.md into index + references

Co-Authored-By: Claude Haiku 4.5 <noreply@anthropic.com>

shared/rules/angular/references/rxjs.md

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+## RxJS
+
+* Use RxJS for asynchronous streams, events and complex reactive flows.
+* Do not use RxJS when simple synchronous signals are sufficient.
+* Prefer declarative observable pipelines.
+* Avoid nested `subscribe`.
+* Avoid manual subscriptions where `async`, signals or framework helpers are sufficient.
+* Prefer `takeUntilDestroyed` or equivalent Angular lifecycle integration for manual subscriptions.
+* Always consider subscription lifetime.
+* Avoid memory leaks from unmanaged subscriptions.
+* Use the correct flattening operator for the required semantics.
+* Do not use `switchMap`, `mergeMap`, `concatMap` or `exhaustMap` interchangeably.
+* Avoid unnecessary `Subject` usage.
+* Prefer exposing observables instead of exposing mutable subjects.
+* Prefer `BehaviorSubject` only when a current value is actually required.
+* Avoid excessive observable chains for simple state.
+* Avoid converting repeatedly between signals and observables without need.