test(live): adr-migrate restores v0 records from snapshots beside the scenario

[aaronsb]

Summary

• Decided: the adr-migrate tier 2 scenario takes its v0 copies of ADR-179 and ADR-186 from snapshots in scenarios/adr-migrate/v0/, taken from main before feat(adr): agent-ways adopts adr/v1 and migrates a first sample #581 (cec7a370^1). Before this, it restored them from the merge-base with main.
• Trades away: the snapshots are frozen. If either record's v0 text needs to change, the snapshot is updated by hand.
• One-way? No.
• Probes: Confident: a local run on this branch passed 10 of 10 (tier 2, adr-migrate only), including a clean lint for ADR-186 migrated under the ADR-305 baseline rule. Not confident: none.
• Inversion: one option restores the records from git history; the other ships them with the test. History isn't available in the release flavor's --depth 1 clone, so they ship with the test.

Why

Today's tier 2 dispatch on main (run 36330173187) failed at setup.sh ran. Main now carries both records as v1, and the setup's guard refuses a rehearsal that would test nothing. In the release flavor, the merge-base lookup had always failed silently. It only passed because main still had the records as v0.