Use goal preview metadata for goal-first threads (#21981)

Fixes #20792

## Why

`/goal`-first threads are valid resumable threads, but they can be
missing from `codex resume` and app recents because discovery depends on
metadata derived from a normal first user message.

PR #21489 attempted to fix this by using the goal objective as
`first_user_message`. Review feedback pointed out that
`first_user_message` does more than provide visible text today: it gates
listing, supplies preview text, and participates in deciding whether a
later title should surface as a distinct thread name. Reusing it for the
goal objective could leave a `/goal`-first thread with
`first_user_message=<goal>` and `title=<later prompt>`, even though the
goal should only provide the initial visible preview.

This PR follows that feedback by and keeps the `first_user_message` as
is but introduces a new `preview` field to separate concerns. The
`preview` field is populated from the first user message or the goal
objective. We can extend it in the future to include other sources.

## What Changed

- Added internal thread `preview` metadata in `codex-state`, including a
SQLite migration that backfills from `first_user_message` and from
existing `thread_goals` objectives when needed.
- Treated `ThreadGoalUpdated` as preview-bearing metadata so goal-first
threads can be listed and searched without mutating
`first_user_message`.
- Updated rollout listing, state queries, thread-store conversion, and
app-server mapping to use preview metadata while continuing to expose
the existing public `preview` field.
- Preserved title/name distinctness behavior around literal
`first_user_message`, so a later normal prompt after `/goal` does not
surface as a separate name just because the goal supplied the initial
preview.
- Preserved compatibility for older/internal metadata writes by deriving
preview from `first_user_message` when explicit preview metadata is
absent.

## Verification

- Manually verified that a thread that starts with a `/goal <objective>`
shows up in the resume picker.

codex-rs/thread-store/src/local/helpers.rs

@@ -110,7 +110,11 @@ pub(super) fn stored_thread_from_rollout_item(
         item.git_origin_url.clone(),
     );
     let source = item.source.unwrap_or(SessionSource::Unknown);
-    let preview = item.first_user_message.clone().unwrap_or_default();
+    let preview = item
+        .preview
+        .clone()
+        .or_else(|| item.first_user_message.clone())
+        .unwrap_or_default();
 
     Some(StoredThread {
         thread_id,

codex-rs/thread-store/src/local/list_threads.rs

@@ -267,6 +267,7 @@ mod tests {
         let mut metadata = builder.build(config.default_model_provider_id.as_str());
         metadata.title = "needle title".to_string();
         metadata.first_user_message = Some("plain preview".to_string());
+        metadata.preview = metadata.first_user_message.clone();
         runtime
             .upsert_thread(&metadata)
             .await

codex-rs/thread-store/src/local/read_thread.rs

@@ -280,11 +280,16 @@ async fn stored_thread_from_sqlite_metadata(
         .ok()
         .map(|meta_line| meta_line.meta);
     let forked_from_id = session_meta.as_ref().and_then(|meta| meta.forked_from_id);
+    let preview = metadata
+        .preview
+        .clone()
+        .or_else(|| metadata.first_user_message.clone())
+        .unwrap_or_default();
     StoredThread {
         thread_id: metadata.id,
         rollout_path: Some(metadata.rollout_path),
         forked_from_id,
-        preview: metadata.first_user_message.clone().unwrap_or_default(),
+        preview,
         name,
         model_provider: if metadata.model_provider.is_empty() {
             store.config.default_model_provider_id.clone()
@@ -1009,8 +1014,9 @@ mod tests {
         builder.cwd = external.path().join("workspace");
         builder.cli_version = Some("sqlite-cli".to_string());
         let mut metadata = builder.build(config.default_model_provider_id.as_str());
-        metadata.title = "SQLite title".to_string();
-        metadata.first_user_message = Some("SQLite preview".to_string());
+        metadata.preview = Some("optimize the benchmark".to_string());
+        metadata.first_user_message = Some("next normal prompt".to_string());
+        metadata.title = "next normal prompt".to_string();
         metadata.model = Some("sqlite-model".to_string());
         runtime
             .upsert_thread(&metadata)
@@ -1028,9 +1034,12 @@ mod tests {
 
         assert_eq!(thread.thread_id, thread_id);
         assert_eq!(thread.rollout_path, Some(rollout_path));
-        assert_eq!(thread.preview, "SQLite preview");
-        assert_eq!(thread.first_user_message.as_deref(), Some("SQLite preview"));
-        assert_eq!(thread.name.as_deref(), Some("SQLite title"));
+        assert_eq!(thread.preview, "optimize the benchmark");
+        assert_eq!(
+            thread.first_user_message.as_deref(),
+            Some("next normal prompt")
+        );
+        assert_eq!(thread.name, None);
         assert_eq!(thread.model_provider, "sqlite-provider");
         assert_eq!(thread.model.as_deref(), Some("sqlite-model"));
         assert_eq!(thread.cwd, external.path().join("workspace"));