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/protocol/src/protocol.rs

@@ -2451,6 +2451,11 @@ impl InitialHistory {
             .and_then(|meta| meta.thread_source)
     }
 
+    pub fn get_resumed_parent_thread_id(&self) -> Option<ThreadId> {
+        self.get_resumed_session_meta()
+            .and_then(|meta| meta.parent_thread_id)
+    }
+
     fn get_resumed_session_meta(&self) -> Option<&SessionMeta> {
         match self {
             InitialHistory::New | InitialHistory::Cleared | InitialHistory::Forked(_) => None,
@@ -2716,6 +2721,8 @@ pub struct SessionMeta {
     pub id: ThreadId,
     #[serde(skip_serializing_if = "Option::is_none")]
     pub forked_from_id: Option<ThreadId>,
+    #[serde(skip_serializing_if = "Option::is_none")]
+    pub parent_thread_id: Option<ThreadId>,
     pub timestamp: String,
     pub cwd: PathBuf,
     pub originator: String,
@@ -2750,6 +2757,7 @@ impl Default for SessionMeta {
         SessionMeta {
             id: ThreadId::default(),
             forked_from_id: None,
+            parent_thread_id: None,
             timestamp: String::new(),
             cwd: PathBuf::new(),
             originator: String::new(),
@@ -3421,6 +3429,8 @@ pub struct SessionConfiguredEvent {
     pub thread_id: ThreadId,
     #[serde(skip_serializing_if = "Option::is_none")]
     pub forked_from_id: Option<ThreadId>,
+    #[serde(skip_serializing_if = "Option::is_none")]
+    pub parent_thread_id: Option<ThreadId>,
     /// Optional analytics source classification for this thread.
     #[serde(default, skip_serializing_if = "Option::is_none")]
     pub thread_source: Option<ThreadSource>,
@@ -3490,6 +3500,7 @@ impl<'de> Deserialize<'de> for SessionConfiguredEvent {
             #[serde(default)]
             thread_id: Option<ThreadId>,
             forked_from_id: Option<ThreadId>,
+            parent_thread_id: Option<ThreadId>,
             #[serde(default)]
             thread_source: Option<ThreadSource>,
             #[serde(default)]
@@ -3530,6 +3541,7 @@ impl<'de> Deserialize<'de> for SessionConfiguredEvent {
             session_id: wire.session_id,
             thread_id: wire.thread_id.unwrap_or_else(|| wire.session_id.into()),
             forked_from_id: wire.forked_from_id,
+            parent_thread_id: wire.parent_thread_id,
             thread_source: wire.thread_source,
             thread_name: wire.thread_name,
             model: wire.model,
@@ -5329,6 +5341,7 @@ mod tests {
                 session_id,
                 thread_id,
                 forked_from_id: None,
+                parent_thread_id: None,
                 thread_source: None,
                 thread_name: None,
                 model: "codex-mini-latest".to_string(),