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/rollout/src/list.rs

@@ -62,6 +62,8 @@ pub struct ThreadItem {
     pub git_origin_url: Option<String>,
     /// Session source from session metadata.
     pub source: Option<SessionSource>,
+    /// Immediate control/spawn parent thread id from session metadata.
+    pub parent_thread_id: Option<ThreadId>,
     /// Random unique nickname from session metadata for AgentControl-spawned sub-agents.
     pub agent_nickname: Option<String>,
     /// Role (agent_role) from session metadata for AgentControl-spawned sub-agents.
@@ -95,6 +97,7 @@ struct HeadTailSummary {
     git_sha: Option<String>,
     git_origin_url: Option<String>,
     source: Option<SessionSource>,
+    parent_thread_id: Option<ThreadId>,
     agent_nickname: Option<String>,
     agent_role: Option<String>,
     model_provider: Option<String>,
@@ -778,6 +781,7 @@ async fn build_thread_item(
             git_sha,
             git_origin_url,
             source,
+            parent_thread_id,
             agent_nickname,
             agent_role,
             model_provider,
@@ -799,6 +803,7 @@ async fn build_thread_item(
             git_sha,
             git_origin_url,
             source,
+            parent_thread_id,
             agent_nickname,
             agent_role,
             model_provider,
@@ -1101,6 +1106,7 @@ async fn read_head_summary(path: &Path, head_limit: usize) -> io::Result<HeadTai
             RolloutItem::SessionMeta(session_meta_line) => {
                 if !summary.saw_session_meta {
                     summary.source = Some(session_meta_line.meta.source.clone());
+                    summary.parent_thread_id = session_meta_line.meta.parent_thread_id;
                     summary.agent_nickname = session_meta_line.meta.agent_nickname.clone();
                     summary.agent_role = session_meta_line.meta.agent_role.clone();
                     summary.model_provider = session_meta_line.meta.model_provider.clone();