migrate-dev-db fails on real data: children of filtered experiment rows are copied #49

vp run migrate-dev-db fails against the real ~/.t3/userdata/statev2.sqlite with MigrateDevDbPhaseError ... during snapshot ... FOREIGN KEY constraint failed, so a worktree cannot be seeded with real data. It fails even with --projects 1 --threads-per-project 3. The source database itself has no foreign-key violations (PRAGMA foreign_key_check is empty).

Cause

copySourceSlice (apps/server/scripts/migrate-dev-db.ts) filters a table to the copied threads only when it has a thread_id column, and copies every other table whole. For the experiment tables that splits parents from children:

• experiment_assignments has thread_id, so it is filtered.
• Its children experiment_outcomes, experiment_observations, experiment_evidence and experiment_evaluations have no thread_id, so every row is copied and points at assignments that were left out.

experiment_work_tasks (filtered) and experiment_work_allocations / experiment_work_evidence (not) have the same shape, and so may external_agent_work with external_agent_sessions.

Acceptance

• vp run migrate-dev-db --base-dir <dir> completes against a database that has experiment assignments belonging to hidden threads.
• A test seeds a parent that is filtered out and a child without thread_id, and shows the child is not copied.
• Found while verifying Make SlipJay workspaces first-class in T3 with a Jujutsu driver #21 in a SlipJay workspace, where the default (worktree) .t3 location is unavailable and --base-dir is needed.