Extension framework post-6856 cleanup (context ownership and package boundaries)

[jongio]

Parent: #6853
Related: #6856, #6835

Objective

Close review-driven cleanup decisions after #6856 without blocking core P1-P3 execution.

Focus items

• F1: azdext.Run vs NewContext ownership/deprecation and migration implications.
• F2: Optional package-boundary split evaluation (azdext/cobra, azdext/mcpgo).

In scope

• Produce explicit decisions and migration plan where needed.
• Track any required follow-up implementation tasks.

Out of scope

• Bundling unrelated feature additions into cleanup.
• Blocking P1-P3 work while design discussion is ongoing.

Deliverables

• Decision record for Run/NewContext ownership.
• If approved, deprecation + migration plan for NewContext.
• Package-boundary evaluation with recommendation and tradeoffs.
• Linked follow-up issues for any approved implementation changes.

Verification checklist

• Decisions are actionable and non-ambiguous.
• Existing extension consumers have a migration path.
• No regressions introduced by cleanup follow-up work.

Definition of done

• F1/F2 decisions documented and tracked to closure.