#102 spec(018): reconcile V2 execution authority with the V3.1 plan

Summary

• Records the V2 to V3.1 migration gate after PR #91 merged to d0015357fd569f25f161168e759037eaeff2f487.
• Preserves CF-01..CF-16, AF history, issue #100, and verified-byte issues #35, #36, #37, #38, and #40 as open until their own evidence exists.
• Does not implement CF-17..CF-28 and does not close any issue.

Test plan

• Exact-head rust, assurance-proof, and scorecard are green on this head.
• af02-base-verifier is green or NOT_APPLICABLE for these docs/spec paths.
• Diff stays limited to specs/018-v3-1-authority-migration/** and the index pointer.
• Post-merge assurance-proof and scorecard are re-read on the merge SHA before this kit is called canonical.

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Summary by cubic

Adds Spec Kit 018-v3-1-authority-migration as the V2 to V3.1 migration gate, reconciling the V3.1 plan with live execution authority without product changes.

Scope

• Keeps CF-01 through CF-16, AF history, issue #100, and verified-byte issues #35 through #38 and #40 open.
• Does not implement CF-17 through CF-28 or close any issue.
• Records PR #102 as the migration candidate identity.
• Requires fresh post-merge assurance-proof and scorecard on the merge SHA before the kit can close.