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/events.rs

@@ -1048,9 +1048,7 @@ pub(crate) fn subagent_thread_started_event_request(
         thread_source: Some(ThreadSource::Subagent),
         initialization_mode: ThreadInitializationMode::New,
         subagent_source: Some(subagent_source_name(&input.subagent_source)),
-        parent_thread_id: input
-            .parent_thread_id
-            .or_else(|| subagent_parent_thread_id(&input.subagent_source)),
+        parent_thread_id: input.parent_thread_id,
         created_at: input.created_at,
     };
     ThreadInitializedEvent {
@@ -1060,19 +1058,7 @@ pub(crate) fn subagent_thread_started_event_request(
 }
 
 pub(crate) fn subagent_source_name(subagent_source: &SubAgentSource) -> String {
-    match subagent_source {
-        SubAgentSource::Review => "review".to_string(),
-        SubAgentSource::Compact => "compact".to_string(),
-        SubAgentSource::ThreadSpawn { .. } => "thread_spawn".to_string(),
-        SubAgentSource::MemoryConsolidation => "memory_consolidation".to_string(),
-        SubAgentSource::Other(other) => other.clone(),
-    }
-}
-
-pub(crate) fn subagent_parent_thread_id(subagent_source: &SubAgentSource) -> Option<String> {
-    subagent_source
-        .parent_thread_id()
-        .map(|parent_thread_id| parent_thread_id.to_string())
+    subagent_source.kind().to_string()
 }
 
 fn analytics_hook_status(status: HookRunStatus) -> HookRunStatus {