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/analytics/src/reducer.rs

@@ -55,7 +55,6 @@ use crate::events::codex_hook_run_metadata;
 use crate::events::codex_plugin_metadata;
 use crate::events::codex_plugin_used_metadata;
 use crate::events::plugin_state_event_type;
-use crate::events::subagent_parent_thread_id;
 use crate::events::subagent_source_name;
 use crate::events::subagent_thread_started_event_request;
 use crate::facts::AnalyticsFact;
@@ -267,20 +266,18 @@ impl ThreadMetadataState {
         session_id: String,
         session_source: &SessionSource,
         thread_source: Option<ThreadSource>,
+        parent_thread_id: Option<String>,
         initialization_mode: ThreadInitializationMode,
     ) -> Self {
-        let (subagent_source, parent_thread_id) = match session_source {
-            SessionSource::SubAgent(subagent_source) => (
-                Some(subagent_source_name(subagent_source)),
-                subagent_parent_thread_id(subagent_source),
-            ),
+        let subagent_source = match session_source {
+            SessionSource::SubAgent(subagent_source) => Some(subagent_source_name(subagent_source)),
             SessionSource::Cli
             | SessionSource::VSCode
             | SessionSource::Exec
             | SessionSource::Mcp
             | SessionSource::Custom(_)
             | SessionSource::Internal(_)
-            | SessionSource::Unknown => (None, None),
+            | SessionSource::Unknown => None,
         };
         Self {
             session_id,
@@ -516,10 +513,7 @@ impl AnalyticsReducer {
         input: SubAgentThreadStartedInput,
         out: &mut Vec<TrackEventRequest>,
     ) {
-        let parent_thread_id = input
-            .parent_thread_id
-            .clone()
-            .or_else(|| subagent_parent_thread_id(&input.subagent_source));
+        let parent_thread_id = input.parent_thread_id.clone();
         let parent_connection_id = parent_thread_id
             .as_ref()
             .and_then(|parent_thread_id| self.threads.get(parent_thread_id))
@@ -1238,6 +1232,7 @@ impl AnalyticsReducer {
         let session_source: SessionSource = thread.source.into();
         let session_id = thread.session_id;
         let thread_id = thread.id;
+        let parent_thread_id = thread.parent_thread_id;
         let Some(connection_state) = self.connections.get(&connection_id) else {
             return;
         };
@@ -1245,6 +1240,7 @@ impl AnalyticsReducer {
             session_id.clone(),
             &session_source,
             thread.thread_source.map(Into::into),
+            parent_thread_id,
             initialization_mode,
         );
         self.threads.insert(