docs(contributing): apply issue-first policy to beta

[ifBars]

Apply the issue-first contribution policy already merged into stable in #340 to beta. Bug fixes, features, and API improvements require a prior GitHub issue and explicit PR linkage; minor documentation-only changes retain the documented exception.

Replaces #341. That PR's correct beta validation and explicit beta-assembly rerun passed, but a separate push-triggered run selected stable game assemblies and left a conflicting required check. This branch uses the existing agent/** push exclusion; the full pull-request validation remains required.

Validation: issue-form YAML and whitespace checks passed. With fresh Steam 0.4.7f7 beta references, S1API builds cleanly in MonoMelon and Il2CppMelon; 740 Mono tests and 720 IL2CPP tests passed. No runtime source or workflows changed.

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