notify: include client in legacy hook payload (#12968)

## Why

The `notify` hook payload did not identify which Codex client started
the turn. That meant downstream notification hooks could not distinguish
between completions coming from the TUI and completions coming from
app-server clients such as VS Code or Xcode. Now that the Codex App
provides its own desktop notifications, it would be nice to be able to
filter those out.

This change adds that context without changing the existing payload
shape for callers that do not know the client name, and keeps the new
end-to-end test cross-platform.

## What changed

- added an optional top-level `client` field to the legacy `notify` JSON
payload
- threaded that value through `core` and `hooks`; the internal session
and turn state now carries it as `app_server_client_name`
- set the field to `codex-tui` for TUI turns
- captured `initialize.clientInfo.name` in the app server and applied it
to subsequent turns before dispatching hooks
- replaced the notify integration test hook with a `python3` script so
the test does not rely on Unix shell permissions or `bash`
- documented the new field in `docs/config.md`

## Testing

- `cargo test -p codex-hooks`
- `cargo test -p codex-tui`
- `cargo test -p codex-app-server
suite::v2::initialize::turn_start_notify_payload_includes_initialize_client_name
-- --exact --nocapture`
- `cargo test -p codex-core` (`src/lib.rs` passed; `core/tests/all.rs`
still has unrelated existing failures in this environment)

## Docs

The public config reference on `developers.openai.com/codex` should
mention that the legacy `notify` payload may include a top-level
`client` field. The TUI reports `codex-tui`, and the app server reports
`initialize.clientInfo.name` when it is available.

codex-rs/hooks/src/user_notification.rs

@@ -1,4 +1,3 @@
-use std::path::Path;
 use std::process::Stdio;
 use std::sync::Arc;
 
@@ -19,6 +18,8 @@ enum UserNotification {
         thread_id: String,
         turn_id: String,
         cwd: String,
+        #[serde(skip_serializing_if = "Option::is_none")]
+        client: Option<String>,
 
         /// Messages that the user sent to the agent to initiate the turn.
         input_messages: Vec<String>,
@@ -28,13 +29,14 @@ enum UserNotification {
     },
 }
 
-pub fn legacy_notify_json(hook_event: &HookEvent, cwd: &Path) -> Result<String, serde_json::Error> {
-    match hook_event {
+pub fn legacy_notify_json(payload: &HookPayload) -> Result<String, serde_json::Error> {
+    match &payload.hook_event {
         HookEvent::AfterAgent { event } => {
             serde_json::to_string(&UserNotification::AgentTurnComplete {
                 thread_id: event.thread_id.to_string(),
                 turn_id: event.turn_id.clone(),
-                cwd: cwd.display().to_string(),
+                cwd: payload.cwd.display().to_string(),
+                client: payload.client.clone(),
                 input_messages: event.input_messages.clone(),
                 last_assistant_message: event.last_assistant_message.clone(),
             })
@@ -56,7 +58,7 @@ pub fn notify_hook(argv: Vec<String>) -> Hook {
                     Some(command) => command,
                     None => return HookResult::Success,
                 };
-                if let Ok(notify_payload) = legacy_notify_json(&payload.hook_event, &payload.cwd) {
+                if let Ok(notify_payload) = legacy_notify_json(payload) {
                     command.arg(notify_payload);
                 }
 
@@ -91,6 +93,7 @@ mod tests {
             "thread-id": "b5f6c1c2-1111-2222-3333-444455556666",
             "turn-id": "12345",
             "cwd": "/Users/example/project",
+            "client": "codex-tui",
             "input-messages": ["Rename `foo` to `bar` and update the callsites."],
             "last-assistant-message": "Rename complete and verified `cargo build` succeeds.",
         })
@@ -102,6 +105,7 @@ mod tests {
             thread_id: "b5f6c1c2-1111-2222-3333-444455556666".to_string(),
             turn_id: "12345".to_string(),
             cwd: "/Users/example/project".to_string(),
+            client: Some("codex-tui".to_string()),
             input_messages: vec!["Rename `foo` to `bar` and update the callsites.".to_string()],
             last_assistant_message: Some(
                 "Rename complete and verified `cargo build` succeeds.".to_string(),
@@ -115,19 +119,27 @@ mod tests {
 
     #[test]
     fn legacy_notify_json_matches_historical_wire_shape() -> Result<()> {
-        let hook_event = HookEvent::AfterAgent {
-            event: crate::HookEventAfterAgent {
-                thread_id: ThreadId::from_string("b5f6c1c2-1111-2222-3333-444455556666")
-                    .expect("valid thread id"),
-                turn_id: "12345".to_string(),
-                input_messages: vec!["Rename `foo` to `bar` and update the callsites.".to_string()],
-                last_assistant_message: Some(
-                    "Rename complete and verified `cargo build` succeeds.".to_string(),
-                ),
+        let payload = HookPayload {
+            session_id: ThreadId::new(),
+            cwd: std::path::Path::new("/Users/example/project").to_path_buf(),
+            client: Some("codex-tui".to_string()),
+            triggered_at: chrono::Utc::now(),
+            hook_event: HookEvent::AfterAgent {
+                event: crate::HookEventAfterAgent {
+                    thread_id: ThreadId::from_string("b5f6c1c2-1111-2222-3333-444455556666")
+                        .expect("valid thread id"),
+                    turn_id: "12345".to_string(),
+                    input_messages: vec![
+                        "Rename `foo` to `bar` and update the callsites.".to_string(),
+                    ],
+                    last_assistant_message: Some(
+                        "Rename complete and verified `cargo build` succeeds.".to_string(),
+                    ),
+                },
             },
         };
 
-        let serialized = legacy_notify_json(&hook_event, Path::new("/Users/example/project"))?;
+        let serialized = legacy_notify_json(&payload)?;
         let actual: Value = serde_json::from_str(&serialized)?;
         assert_eq!(actual, expected_notification_json());