store and expose parent_thread_id on Threads (#25113)

## Why

This PR
https://github.com/openai/codex/pull/24161#discussion_r3325692763
revealed a subagent data modeling issue, where we overloaded
`forked_from_id` to also mean `parent_thread_id`. That's incorrect since
guardian and review subagents can be a subagent and NOT fork the main
thread's history.

The solution here is to explicitly store a new `parent_thread_id` on
`SessionMeta`, alongside `forked_from_id` which already exists. While
we're at it, also expose it in the app-server protocol on the `Thread`
object.

A thread->subagent relationship and a fork of thread history are
orthogonal concepts.

## What Changed

- Added top-level `parent_thread_id` persistence on `SessionMeta` and
runtime/session plumbing through `SessionConfiguredEvent`,
`CodexSpawnArgs`, `SessionConfiguration`, `ThreadConfigSnapshot`,
`TurnContext`, and `ModelClient`.
- Made turn metadata, request headers, analytics, and subagent-start
events read the separate runtime/top-level parent field instead of
deriving general parent lineage from `SessionSource` or
`forked_from_thread_id`.
- Passed parent lineage separately at delegated subagent, review,
guardian, agent-job, and multi-agent spawn construction sites;
copied-history fork lineage remains derived only from `InitialHistory`.
- Persisted and exposed parent lineage through rollout/thread-store
projections and app-server v2 `Thread.parentThreadId`.
- Updated app-server README text and regenerated app-server schema
fixtures for the additive `parentThreadId` response field.

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

@@ -235,6 +235,7 @@ async fn read_thread_from_rollout_path(
     })?;
     if let Ok(meta_line) = read_session_meta_line(path.as_path()).await {
         thread.forked_from_id = meta_line.meta.forked_from_id;
+        thread.parent_thread_id = meta_line.meta.parent_thread_id;
         if let Some(model_provider) = meta_line
             .meta
             .model_provider
@@ -287,6 +288,7 @@ 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 parent_thread_id = session_meta.as_ref().and_then(|meta| meta.parent_thread_id);
     let preview = metadata
         .preview
         .clone()
@@ -298,6 +300,7 @@ async fn stored_thread_from_sqlite_metadata(
         thread_id: metadata.id,
         rollout_path: Some(metadata.rollout_path),
         forked_from_id,
+        parent_thread_id,
         preview,
         name,
         model_provider: if metadata.model_provider.is_empty() {
@@ -361,6 +364,7 @@ fn stored_thread_from_meta_line(
         thread_id: meta_line.meta.id,
         rollout_path: Some(path),
         forked_from_id: meta_line.meta.forked_from_id,
+        parent_thread_id: meta_line.meta.parent_thread_id,
         preview: String::new(),
         name: None,
         model_provider: meta_line