core: persist initial context window metadata (#29519)

## Why

PR #29494 made context-window IDs visible to the model by wrapping the
token-budget window payload in `<context_window>`, but rollout JSONL
consumers still could not see the initial window identity by tailing the
session file. Compacted rollout items carry window IDs only after
compaction has happened, so a session with no compaction had no durable
JSONL record for window 0.

This change gives tailing consumers a stable initial-window record at
session creation time.

## What Changed

- Added `session_meta.context_window.window_id` for the initial
context-window identity.
- `CreateThreadParams` now requires `initial_window_id: String`, so
thread-store callers cannot accidentally create new threads without
window-0 metadata.
- Live thread creation derives the persisted initial window ID from the
same `AutoCompactWindowIds` used to initialize `SessionState`, keeping
runtime state and JSONL metadata aligned.
- Rollout reconstruction uses `session_meta.context_window.window_id` as
the initial-window fallback and derives `window_number = 0`,
`first_window_id = window_id`, and `previous_window_id = None`
internally.
- Fork reconstruction intentionally uses the same rollout reconstruction
path; consumers that need to distinguish copied initial-window metadata
can use the rollout `thread_id`.
- Legacy compactions without `window_number` still use compaction-count
fallback accounting instead of being reset to window 0 by the
initial-window fallback.
- Compacted rollout metadata still takes precedence once compaction
records exist, preserving the richer chain fields there.

## JSONL Shape

Real rollout JSONL is one object per line. This example is expanded for
readability, but shows the new initial `session_meta.context_window`
record followed by the existing compacted rollout item shape that also
carries window IDs:

```jsonl
{
  "timestamp": "2026-06-22T12:00:00.000Z",
  "type": "session_meta",
  "payload": {
    "session_id": "<THREAD_ID>",
    "id": "<THREAD_ID>",
    "timestamp": "2026-06-22T12:00:00.000Z",
    "cwd": "/repo",
    "originator": "codex",
    "cli_version": "0.0.0",
    "source": "cli",
    "model_provider": "<MODEL_PROVIDER>",
    "context_window": {
      "window_id": "<INITIAL_WINDOW_ID>"
    }
  }
}
...
{
  "timestamp": "2026-06-22T12:34:56.000Z",
  "type": "compacted",
  "payload": {
    "message": "<COMPACTION_SUMMARY>",
    "replacement_history": [
      "..."
    ],
    "window_number": 1,
    "first_window_id": "<INITIAL_WINDOW_ID>",
    "previous_window_id": "<INITIAL_WINDOW_ID>",
    "window_id": "<NEXT_WINDOW_ID>"
  }
}
```

The nested `context_window` object is intentional: it gives rollout
consumers a stable namespace for context-window metadata while only
writing the non-derivable initial `window_id`. For the initial window,
`window_number`, `first_window_id`, and `previous_window_id` are derived
internally instead of being written to the rollout.

## 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`
- `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`

codex-rs/app-server/src/request_processors/external_agent_session_import.rs

@@ -215,6 +215,7 @@ impl ExternalAgentSessionImporter {
             },
             dynamic_tools: Vec::new(),
             multi_agent_version: Some(MultiAgentVersion::V1),
+            initial_window_id: uuid::Uuid::now_v7().to_string(),
             metadata: ThreadPersistenceMetadata {
                 cwd: Some(cwd.clone()),
                 model_provider: model_provider.clone(),

codex-rs/app-server/tests/common/rollout.rs

@@ -202,6 +202,7 @@ fn create_fake_rollout_with_source_and_parent_thread_id(
         dynamic_tools: None,
         memory_mode: None,
         multi_agent_version: None,
+        context_window: None,
     };
     let payload = serde_json::to_value(SessionMetaLine {
         meta,
@@ -289,6 +290,7 @@ pub fn create_fake_rollout_with_text_elements(
         dynamic_tools: None,
         memory_mode: None,
         multi_agent_version: None,
+        context_window: None,
     };
     let payload = serde_json::to_value(SessionMetaLine {
         meta,

codex-rs/app-server/tests/suite/conversation_summary.rs

@@ -131,6 +131,7 @@ async fn get_conversation_summary_by_thread_id_reads_pathless_store_thread() ->
             base_instructions: BaseInstructions::default(),
             dynamic_tools: Vec::new(),
             multi_agent_version: None,
+            initial_window_id: Uuid::now_v7().to_string(),
             metadata: ThreadPersistenceMetadata {
                 cwd: None,
                 model_provider: "test-provider".to_string(),

codex-rs/app-server/tests/suite/v2/remote_thread_store.rs

@@ -157,6 +157,7 @@ async fn thread_delete_with_non_local_thread_store_does_not_create_local_persist
             base_instructions: BaseInstructions::default(),
             dynamic_tools: Vec::new(),
             multi_agent_version: None,
+            initial_window_id: Uuid::now_v7().to_string(),
             metadata: ThreadPersistenceMetadata {
                 cwd: Some(codex_home.path().to_path_buf()),
                 model_provider: "mock_provider".to_string(),

codex-rs/app-server/tests/suite/v2/thread_read.rs

@@ -1367,6 +1367,7 @@ async fn seed_pathless_store_thread(
             base_instructions: BaseInstructions::default(),
             dynamic_tools: Vec::new(),
             multi_agent_version: None,
+            initial_window_id: Uuid::now_v7().to_string(),
             metadata: ThreadPersistenceMetadata {
                 cwd: None,
                 model_provider: "test-provider".to_string(),

codex-rs/app-server/tests/suite/v2/thread_resume.rs

@@ -2072,6 +2072,7 @@ stream_max_retries = 0
         dynamic_tools: None,
         memory_mode: None,
         multi_agent_version: None,
+        context_window: None,
     };
     std::fs::write(
         &rollout_path,

codex-rs/app-server/tests/suite/v2/thread_unarchive.rs

@@ -218,6 +218,7 @@ async fn thread_unarchive_preserves_pathless_store_metadata() -> Result<()> {
             base_instructions: BaseInstructions::default(),
             dynamic_tools: Vec::new(),
             multi_agent_version: None,
+            initial_window_id: Uuid::now_v7().to_string(),
             metadata: ThreadPersistenceMetadata {
                 cwd: None,
                 model_provider: "test-provider".to_string(),