[Sprint 7] docs(adr): record xUnit v3 migration decision + per-project rollout plan

[mpaulosky]

Goal

Record the xUnit v3 migration decision in an ADR and link it from the docs index. Document the per-project rollout strategy and coexistence approach via Directory.Packages.props.

Context

Part of the xUnit v3 migration pilot (Sprints 7–13). The pilot validates the migration process on tests/Domain.Tests before any other project is touched. Plan thread: see Sprint 7 milestone.

Acceptance Criteria

• New ADR file in docs/decisions/ (e.g. ADR-002-xunit-v3-migration.md)
• ADR includes: context, decision, alternatives, consequences, rollout plan (Sprints 7–13)
• Linked from docs/decisions/index.md
• Cross-referenced in AGENTS.md AI-Agent Notes if material to agent workflow

References

• Migration guide: https://xunit.net/docs/getting-started/v3/migration
• What's new: https://xunit.net/docs/getting-started/v3/whats-new
• MTP: https://xunit.net/docs/getting-started/v3/microsoft-testing-platform

Squad

Pippin (Docs) + Aragorn (Lead review)

[github-actions]

📋 Assigned to Pippin (Docs)

Issue: #166 — [Sprint 7] docs(adr): record xUnit v3 migration decision + per-project rollout plan

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