Add rollout migration tooling and background migration (#37348)

## What changed

- Add `codex migrate-rollouts` with dry-run inspection by default, explicit `--apply`, thread filtering, optional I/O throttling, progress output, and JSON or verbose reports.
- Add the disabled-by-default `background_paginated_rollout_migration` feature to migrate legacy local sessions at startup before rollout compression begins.
- Persist a migration cursor and skipped-file fingerprints so later startups avoid full rescans while retrying changed files and recovering pending migrations.
- Coordinate migration with live writers and emit metrics for manual and startup runs.

## Testing

- Cover startup cursor advancement and lookback, pending migration recovery, live-writer coordination, changed empty rollouts, and progress reporting.

GitOrigin-RevId: 276ac506c50ebec5140fd319faca1bb998172061

codex-rs/thread-store/src/lib.rs

@@ -27,6 +27,7 @@ pub use local::LocalThreadStoreConfig;
 pub use local::RolloutMigrationMode;
 pub use local::RolloutMigrationOptions;
 pub use local::RolloutMigrationOutcome;
+pub use local::RolloutMigrationProgress;
 pub use local::RolloutMigrationReport;
 pub use local::RolloutMigrationStatus;
 pub use queue_store::LocalQueueStore;