## Why
Rollout migration outcomes expose free-form error messages, making failures hard to group without parsing text.
## What changed
- Add a serialized `RolloutMigrationFailureReason` to failed outcomes and classify failures across reading, conversion, SQLite materialization, publishing, and recovery.
- Add the failure reason as a low-cardinality tag on per-thread migration telemetry.
## Testing
- Cover missing SQLite metadata and invalid session metadata classifications.
GitOrigin-RevId: 2bff98483b484f4d02b72d52f39950ff99ebe429
## 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
Owen Lin