Fix store migration so the live database moves to the new scheme

docs/plan.md

@@ -451,6 +451,7 @@ Issues identified in review that implementation must account for:
 - **Squash-and-reset under a running agent.** Agents may notice their commits disappearing and try to repair history; step 7 of the merge cycle addresses this and needs testing per vendor.
 - **Event log growth.** Stream-json is verbose and tool outputs are large. SSE resume and scrubbing need efficient queries; consider separating large payloads into blobs.
 - **Migration numbering across branches.** Migrations are tracked by position (`PRAGMA user_version`). Each task branch appends its own, and byre is deployed from task branches, so two branches can claim the same number: the database records it as applied and the other branch's migration silently never runs. Fix: track applied migrations by name in a table. Before switching, make every existing data migration safe to run twice, since one recorded only by number would run again under its name.
+  - **Status: fixed.** Migrations are named and tracked in a `migrations` table (see `docs/operations.md`, "Data").
 
 ---