core: persist initial context window metadata

Record the initial AutoCompactWindowIds window_id in session_meta.context_window so JSONL tailers can observe the first context window before any compaction item exists. Thread creation now requires an initial_window_id string, while rollout JSONL keeps that value under the context_window namespace without writing derived chain fields for the initial window.

Rollout reconstruction derives window 0 state from that initial_window_id when no compaction window metadata exists. Forks intentionally use the same reconstruction path; consumers that need to distinguish copied initial-window metadata can use thread_id. Legacy compactions without window_number continue to use compaction-count fallback accounting.

Verification:
- just test -p codex-protocol
- just test -p codex-rollout recorder_materializes_on_flush_with_pending_items
- just test -p codex-core reconstruct_history_restores_initial_window_from_session_meta
- just test -p codex-core reconstruct_history_prefers_compacted_window_over_session_meta
- just test -p codex-core reconstruct_history_preserves_legacy_compaction_count_with_session_meta_window
- just test -p codex-core reconstruct_history
- just test -p codex-core record_initial_history_reconstructs_forked_transcript
- just test -p codex-thread-store
- just test -p codex-state
- just test -p codex-app-server thread_read_returns_summary_without_turns
- just test -p codex-rollout persistence_metrics
- just fix -p codex-protocol -p codex-rollout -p codex-thread-store -p codex-core -p codex-app-server
- just fix -p codex-core -p codex-rollout
- just fix -p codex-core

codex-rs/thread-store/src/in_memory.rs

@@ -10,6 +10,7 @@ use codex_protocol::ThreadId;
 use codex_protocol::models::PermissionProfile;
 use codex_protocol::protocol::AskForApproval;
 use codex_protocol::protocol::RolloutItem;
+use codex_protocol::protocol::SessionContextWindow;
 use codex_protocol::protocol::SessionMeta;
 use codex_protocol::protocol::SessionMetaLine;
 use codex_protocol::protocol::ThreadMemoryMode;
@@ -120,6 +121,7 @@ mod tests {
                     base_instructions: BaseInstructions::default(),
                     dynamic_tools: Vec::new(),
                     multi_agent_version: None,
+                    initial_window_id: uuid::Uuid::now_v7().to_string(),
                     metadata: ThreadPersistenceMetadata {
                         cwd: None,
                         model_provider: "test-provider".to_string(),
@@ -248,6 +250,7 @@ impl InMemoryThreadStore {
             memory_mode: matches!(params.metadata.memory_mode, ThreadMemoryMode::Disabled)
                 .then_some("disabled".to_string()),
             multi_agent_version: params.multi_agent_version,
+            context_window: Some(SessionContextWindow::new(params.initial_window_id.clone())),
             ..SessionMeta::default()
         };
         state

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

@@ -37,7 +37,8 @@ pub(super) async fn create_thread(
             params.dynamic_tools,
         )
         .with_session_id(params.session_id)
-        .with_multi_agent_version(params.multi_agent_version),
+        .with_multi_agent_version(params.multi_agent_version)
+        .with_initial_window_id(params.initial_window_id),
     )
     .await
     .map_err(|err| ThreadStoreError::Internal {

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

@@ -1133,6 +1133,7 @@ mod tests {
             base_instructions: BaseInstructions::default(),
             dynamic_tools: Vec::new(),
             multi_agent_version: None,
+            initial_window_id: uuid::Uuid::now_v7().to_string(),
             metadata: thread_metadata(),
         }
     }

codex-rs/thread-store/src/types.rs

@@ -85,6 +85,8 @@ pub struct CreateThreadParams {
     pub dynamic_tools: Vec<DynamicToolSpec>,
     /// Multi-agent runtime selected when the thread was created.
     pub multi_agent_version: Option<MultiAgentVersion>,
+    /// Initial context-window identity captured when the thread was created.
+    pub initial_window_id: String,
     /// Metadata captured for the newly created thread.
     pub metadata: ThreadPersistenceMetadata,
 }