refactor(core): move the schema migrations into config/migrate.rs

`config.rs` is 1057 lines, past the 1000-line limit, and 884 of them are one `impl Config` block. The three load-time migration passes and the owner inference only the first of them uses are 250 of those lines, and nothing else in the block calls them: `file.rs` runs them while parsing, and the tests drive `migrate_transport_scoped_keys` directly. They move verbatim into an `impl Config` block of their own in `config/migrate.rs`. `migrate_owner_locked_gestures` and `migrate_thumbwheel_native_direction` become `pub(super)`, because `file.rs` is now a sibling of the module that defines them rather than a descendant, the way `repair_duplicate_routes` in `identity.rs` already is; narrowing either back breaks the build. `DeviceConfig` is named only in the moved docs, so its two links gain reference definitions rather than an import nothing would use.