Add avoid_context_read_in_build lint

[cupofme]

This PR adds a lint for easy to overlook bug: reading a value with context.read inside build method. read takes a one-time snapshot and never re-subscribes, so the UI quietly goes stale — reasonable-looking code that's easy to miss in review and annoying to debug later.

Now the analyzer flags it the moment it's written and offers a one-click fix to swap read for watch. It stays quiet on all the legitimate uses — calling a method, adding an event, grabbing a bloc reference, or reading inside a callback.

// BAD — reads a snapshot, UI won't rebuild when it changes
final count = context.read<CounterCubit>().state;
return Text('$count');

// GOOD — watch keeps the widget in sync
final count = context.watch<CounterCubit>().state;
return Text('$count');

// GOOD — reads that aren't build data stay quiet
onPressed: () => context.read<CounterCubit>().increment(),

read.watch.mov

[PiotrRogulski]

I'm not sure if this is OK – it's still top-level read in build. I feel like we should flag all reads, no matter what the purpose is 🤔

Invoking a method is still using the value

[cupofme]

Now all build-methods reads are flagged except for ones that are used in callbacks

[PiotrRogulski]

One last thing 🙏

[PiotrRogulski]

select() is also valid (just like BlocSelector). We probably can't suggest it in the fix (because it would require rewriting more code), but we should mention it in docs and messages