Branching: preview `migrate` step DIES on multi-migration gap; `supabase db push --include-all` applies the same gap cleanly

## Summary When a Supabase preview branch is created (either via GitHub integration on a PR or via Management API / MCP), it spawns with a **subset** of the parent project's migration history applied to the tracking table, then the preview-branching `migrate` step DIES trying to apply the gap. The same gap is applied without error by `supabase db push --include-all --linked` from the CLI against the same preview branch. This generates spurious "⚠️ Service health check failed" comments on PR previews even though all services are healthy and our migrations are valid. ## Reproduction Tested against project ref `byvleunmmqjfroztnqgy` (parent) on 2026-04-22. 1. Parent project has 25 migrations tracked in `supabase_migrations.schema_migrations`, earliest `20260417000000` (post-consolidation baseline), latest `20260422021351`. All migrations are committed and confirmed in sync via `supabase migration list --linked`. 2. Created a branch via Management API: ```bash curl -X POST -H "Authorization: Bearer $SUPABASE_TOKEN" \ "https://api.supabase.com/v1/projects/byvleunmmqjfroztnqgy/branches" \ -d '{"branch_name":"health-probe-investigation"}' # → branch ref: cfwtrulhimxjapemejyd ``` 3. Polled `/v1/projects/{preview_ref}/actions`: ``` clone: EXITED ✅ pull: EXITED ✅ health: EXITED ✅ configure: EXITED ✅ migrate: DEAD ❌ seed: CREATED (skipped) deploy: CREATED (skipped) ``` 4. Queried preview's tracking table: ```sql SELECT COUNT(*), MIN(version), MAX(version) FROM supabase_migrations.schema_migrations; -- 9, 20260417000000, 20260419010459 ``` Preview was tracking only 9 of the parent's 25 migrations — missing the latest 16. 5. Verified missing-migration artifacts were absent: tables/enums introduced by the 16 missing migrations did NOT exist on the preview, and a column dropped by one of them was still present. Preview's data state was consistent with its tracking — i.e., the gap mi…