docs: amend ADR 0002 for #676's bounded competency criteria reads

[mgwozdz-unicon]

This amends ADR 0002 (Decision 2) per discussion on #676. #676's concern is that an unscoped read of a heavily-reused competency's criteria can grow unbounded as course runs accumulate over time, so it adds a read path that resolves scope from an explicit, capped set of course keys rather than by course-run date overlap. The existing ADR language didn't account for that read path.

• course_id: clarifies that every group in a course-scoped subtree carries the same course_id, not only its topmost group, and that the field is immutable after creation. This was previously ambiguous, and [ARCH] Plan for scaling Competency Criteria Group associations across recurring course runs #676's query plan for that panel depends on it holding for every group in the subtree, not just the top one.
• Retrieval scope: drops the date-window-specific formula, since [ARCH] Plan for scaling Competency Criteria Group associations across recurring course runs #676 introduces a read path that doesn't window by dates at all. The two underlying invariants (root scope always included, a selected subtree always returned in full) are restated without prescribing how the scoped set is resolved.
• Pagination-support note: removed, since it's no longer accurate once a response can never split a subtree across pages; [ARCH] Plan for scaling Competency Criteria Group associations across recurring course runs #676 bounds request size instead of paginating the response.

Related to #676

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