Expand description
Bootstrap: stamp all pre-framework migrations as applied for existing users.
Any user who has launched AgentMux before the migration framework existed
will have the old startup-embedded migrations already applied (via the
marker-file system). This step reads those marker files and stamps the
corresponding migration IDs into db_migrations so the runner skips them.
For migrations without a deterministic marker file, we check for the presence of the data they produce. If we cannot determine whether a migration ran, we leave it unstamped so it runs (all ported migrations are idempotent and safe to re-run).
This migration itself is idempotent: on second run the db_migrations rows
are already present and migration_is_applied returns true before we even
get here.