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/features/src/lib.rs

@@ -147,6 +147,8 @@ pub enum Feature {
     ExternalAgentMemoryImport,
     /// Compress cold local thread-store rollout files.
     LocalThreadStoreCompression,
+    /// Migrate legacy local rollout files to paginated history in the background.
+    BackgroundPaginatedRolloutMigration,
     /// Enable the Chronicle sidecar for passive screen-context memories.
     Chronicle,
     /// Compress request bodies (zstd) when sending streaming requests to codex-backend.
@@ -1009,6 +1011,12 @@ pub const FEATURES: &[FeatureSpec] = &[
         stage: Stage::UnderDevelopment,
         default_enabled: false,
     },
+    FeatureSpec {
+        id: Feature::BackgroundPaginatedRolloutMigration,
+        key: "background_paginated_rollout_migration",
+        stage: Stage::UnderDevelopment,
+        default_enabled: false,
+    },
     FeatureSpec {
         id: Feature::Chronicle,
         key: "chronicle",