feat(codex): migrate third-party history provider bucket to unified "custom" ID

Codex CLI filters conversations by model_provider — after normalizing
all third-party providers to "custom", old sessions with their original
provider IDs become invisible. This migration rewrites JSONL session
files and state_5.sqlite threads to use the unified ID.

Uses a three-tier trust model for source ID collection:
- Known legacy preset whitelist (46 IDs, case-insensitive)
- cc-switch-created custom providers (identified by created_at)
- User-maintained custom providers are explicitly skipped

Adds scanned_history_files flag to prevent stale v1 markers from
blocking re-runs with the improved source ID collection logic.