fix(codex): recover legacy thread names

[huzhongyyuan]

Closes #395.

Backfills legacy Codex thread names from session_index.jsonl into blank threads.name values during startup, allowing native Codex CLI and VS Code surfaces to retain renamed session titles after provider/relay changes.

Safety properties:

• never overwrites non-empty native names;
• updates only threads.name, preserving archive and unrelated state;
• supports the default state DB and configured sqlite_home;
• uses BEGIN IMMEDIATE, source snapshot/digest checks, a fresh SQLite backup, and a per-DB transactional ledger;
• retries safely after interruption, handles DB replacement, isolates per-DB failures, and bounds index/DB work.

Validation:

• cargo fmt --check
• cargo test codex_history_migration::tests:: --lib (52 passed)
• cargo test settings::tests:: --lib (19 passed)
• git diff --check

Repository-wide cargo clippy remains blocked on unchanged src/cli/tui/ui/home_chart.rs:806 (reversed_empty_ranges). Full integration-test compilation also has unchanged test-harness errors in tests/settings_current_provider.rs and tests/settings_visible_apps.rs; these are not introduced by this patch.

Related upstream Codex reports: openai/codex#16405 and openai/codex#16280.