[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/app-server-protocol/src/protocol/v2/tests.rs

@@ -174,6 +174,7 @@ fn thread_resume_response_round_trips_initial_turns_page() {
             model_provider: "openai".to_string(),
             created_at: 1,
             updated_at: 1,
+            recency_at: Some(1),
             status: ThreadStatus::Idle,
             path: None,
             cwd: absolute_path("tmp"),
@@ -3600,6 +3601,7 @@ fn thread_lifecycle_responses_default_missing_optional_fields() {
 
     assert_eq!(start.instruction_sources, Vec::<AbsolutePathBuf>::new());
     assert_eq!(start.thread.parent_thread_id, None);
+    assert_eq!(start.thread.recency_at, None);
     assert_eq!(resume.instruction_sources, Vec::<AbsolutePathBuf>::new());
     assert_eq!(fork.instruction_sources, Vec::<AbsolutePathBuf>::new());
     assert_eq!(start.active_permission_profile, None);
@@ -3608,6 +3610,14 @@ fn thread_lifecycle_responses_default_missing_optional_fields() {
     assert_eq!(fork.active_permission_profile, None);
 }
 
+#[test]
+fn thread_recency_sort_key_serializes_as_snake_case() {
+    assert_eq!(
+        serde_json::to_value(ThreadSortKey::RecencyAt).expect("sort key should serialize"),
+        json!("recency_at")
+    );
+}
+
 #[test]
 fn turn_start_params_preserve_explicit_null_service_tier() {
     let params: TurnStartParams = serde_json::from_value(json!({

codex-rs/app-server-protocol/src/protocol/v2/thread.rs

@@ -1086,6 +1086,7 @@ pub enum ThreadSourceKind {
 pub enum ThreadSortKey {
     CreatedAt,
     UpdatedAt,
+    RecencyAt,
 }
 
 #[derive(Serialize, Deserialize, Debug, Clone, Copy, PartialEq, JsonSchema, TS)]

codex-rs/app-server-protocol/src/protocol/v2/thread_data.rs

@@ -152,6 +152,9 @@ pub struct Thread {
     /// Unix timestamp (in seconds) when the thread was last updated.
     #[ts(type = "number")]
     pub updated_at: i64,
+    /// Unix timestamp (in seconds) used for thread recency ordering.
+    #[ts(type = "number | null")]
+    pub recency_at: Option<i64>,
     /// Current runtime status for the thread.
     pub status: ThreadStatus,
     /// [UNSTABLE] Path to the thread on disk.