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/core/src/thread_manager.rs

@@ -349,16 +349,32 @@ pub fn thread_store_from_config(
 ) -> Arc<dyn ThreadStore> {
     match &config.experimental_thread_store {
         ThreadStoreConfig::Local => {
-            if config
+            let compression_enabled = config
                 .features
-                .enabled(Feature::LocalThreadStoreCompression)
-            {
-                codex_rollout::spawn_rollout_compression_worker(config.codex_home.to_path_buf());
-            }
-            Arc::new(LocalThreadStore::new(
+                .enabled(Feature::LocalThreadStoreCompression);
+            let background_migration_enabled = config
+                .features
+                .enabled(Feature::BackgroundPaginatedRolloutMigration);
+            let has_state_db = state_db.is_some();
+            let store = Arc::new(LocalThreadStore::new(
                 LocalThreadStoreConfig::from_config(config),
                 state_db,
-            ))
+            ));
+            if has_state_db && background_migration_enabled {
+                let startup_store = Arc::clone(&store);
+                let codex_home = config.codex_home.to_path_buf();
+                tokio::spawn(async move {
+                    if let Err(err) = startup_store.migrate_rollouts_on_startup().await {
+                        warn!("failed to migrate legacy rollouts on startup: {err}");
+                    }
+                    if compression_enabled {
+                        codex_rollout::spawn_rollout_compression_worker(codex_home);
+                    }
+                });
+            } else if compression_enabled {
+                codex_rollout::spawn_rollout_compression_worker(config.codex_home.to_path_buf());
+            }
+            store
         }
         ThreadStoreConfig::InMemory { id } => InMemoryThreadStore::for_id(id),
     }