[codex] Restore thread recency with compatible migration history (#28671)

## Summary

- Revert #28655, restoring the thread `recencyAt` behavior introduced by
#27910.
- Move `threads_recency_at` to migration 0039 so it no longer collides
with `external_agent_config_imports` at version 0038.
- Repair databases that already applied the recency migration as version
38 by moving the matching migration-history row to version 39 before
SQLx validation. The current version-38 migration can then apply
normally.

## Validation

- `just test -p codex-state
migrations::tests::repairs_recency_migration_that_was_applied_as_version_38`
- `just test -p codex-state -p codex-rollout -p codex-thread-store -p
codex-app-server-protocol -p codex-tui`: 3,439 passed; six TUI tests
could not open the machine's existing read-only incident database at
`~/.codex/sqlite/state_5.sqlite`.
- `just fix -p codex-state`
- `just fmt`
- Verified that state migration versions are unique.

codex-rs/exec/src/lib_tests.rs

@@ -342,6 +342,7 @@ fn turn_items_for_thread_returns_matching_turn_items() {
         model_provider: "openai".to_string(),
         created_at: 0,
         updated_at: 0,
+        recency_at: Some(0),
         status: codex_app_server_protocol::ThreadStatus::Idle,
         path: None,
         cwd: test_path_buf("/tmp/project").abs(),
@@ -760,6 +761,7 @@ fn sample_thread_start_response() -> ThreadStartResponse {
             model_provider: "openai".to_string(),
             created_at: 0,
             updated_at: 0,
+            recency_at: Some(0),
             status: codex_app_server_protocol::ThreadStatus::Idle,
             path: Some(PathBuf::from("/tmp/rollout.jsonl")),
             cwd: test_path_buf("/tmp").abs(),