Add open question on contractual data-use restrictions and merge scope

[RobGrafrath]

What this changes and why

Community discussion of the HIVE-0002 worked example raised a question the record does not cover: contractual restrictions on data co-mingling, where a client forbids its account data from being used for other accounts. Entity matching and the golden record currently compose across the whole database, and some client contracts forbid exactly that. This adds a sixteenth entry to OPEN-QUESTIONS.md, Contractual data-use restrictions and merge scope, with its tracking issue #20 already filed and labeled open-question.

Interacting decisions

• HIVE-0002 Entity Matching, Golden Record, and Locale-Aware Identity Resolution: the open question qualifies its database-wide merge scope; no change to the decision in this pull request.
• HIVE-0003 Demographic History, Golden Record, and Contact Intelligence: provenance carrying usage rights would extend its per-value source records; no change in this pull request.

No decision is contradicted; the entry records that the question is open.

Version bumps

None. OPEN-QUESTIONS.md is not a decision file, and no decision file changes.

Checklist

• I read the decisions neighboring this change, not just the file I edited.
• New or retitled decisions have a matching line in INDEX.md, and front matter follows CONTRIBUTING.md.
• python3 scripts/validate.py --base main passes locally, or I am relying on CI to tell me.
• No pricing, rates, market or customer allocation, consumer data, or client-confidential information.
• Claims about law or regulation are flagged for verification, not asserted.
• Commits are signed off (DCO). If I forgot, I will post the retroactive sign-off comment per CONTRIBUTING.md.
• I license this contribution under CC BY 4.0 (prose) and Apache 2.0 (embedded code samples).