Subscribe authors to message-board channels created by posting (#47259)

## Why

Posting to a new channel subscribed its author to replies on that post, but not to new discussion roots in the channel. The creator could therefore miss notifications for subsequent discussions.

## What changed

Subscribe the author to the channel in the same transaction that creates the channel and its first post. Document this behavior in the API and the `new_channel_name` tool parameter.

## Testing

Update coverage to verify that channel creators receive notifications without explicit targeting and that posting to an existing channel preserves an explicit unsubscribe. Adjust thread-deletion test expectations for the additional persisted subscription.

GitOrigin-RevId: 5553ebc9f9b51456ab48886e9871733e93370a2b

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

@@ -391,7 +391,7 @@ async fn thread_delete_removes_persisted_board_even_with_feature_disabled() -> R
             /*busy_timeout*/ None,
         )
         .await?;
-    assert_eq!(board_counts(&pool).await?, (1, 1, 1));
+    assert_eq!(board_counts(&pool).await?, (1, 1, 2));
     let _: ThreadArchiveResponse = app
         .request(|request_id| ClientRequest::ThreadArchive {
             request_id,
@@ -409,7 +409,7 @@ async fn thread_delete_removes_persisted_board_even_with_feature_disabled() -> R
         .with_codex_home(home.path())
         .build_initialized()
         .await?;
-    assert_eq!(board_counts(&pool).await?, (1, 1, 1));
+    assert_eq!(board_counts(&pool).await?, (1, 1, 2));
     let _: ThreadDeleteResponse = app
         .request(|request_id| ClientRequest::ThreadDelete {
             request_id,

codex-rs/ext/agent-message-board/src/api.rs

@@ -51,6 +51,7 @@ pub trait AgentMessageBoard: Send + Sync {
     /// Uses the caller's configured clock; clock failures must not create a post.
     /// The request ID identifies a logical call across retries. A retry with
     /// different input is an error; a successful retry returns the same metadata.
+    /// Creating a channel while posting also subscribes its author to new roots there.
     fn post(&self, caller: ThreadId, request: PostRequest) -> BoxFuture<'_, Result<PostMetadata>>;
 
     fn list_threads(

codex-rs/ext/agent-message-board/src/local.rs

@@ -235,6 +235,12 @@ impl LocalAgentMessageBoard {
             PostDestination::NewChannel(channel) => {
                 validate_channel(channel)?;
                 self.insert_channel(&mut tx, channel, &author, now).await?;
+                self.subscribe(
+                    &mut tx,
+                    &SubscriptionTarget::Channel(channel.clone()),
+                    caller,
+                )
+                .await?;
                 (
                     channel.clone(),
                     id,

codex-rs/ext/agent-message-board/src/tools/spec.rs

@@ -87,6 +87,9 @@ pub(super) fn tool(name: &str, namespace: Option<&str>, namespace_description: &
     let mut properties = serde_json::Map::new();
     for field in fields {
         let schema = match *field {
+            "new_channel_name" => {
+                json!({"type":"string","description":"Create and subscribe to this channel."})
+            }
             "subscribe" => {
                 json!({"type":"boolean","description":"Subscribe to new roots. Default true."})
             }

codex-rs/ext/agent-message-board/tests/local_board.rs

@@ -102,11 +102,13 @@ async fn shared_handles_resume_posts_and_preserve_subscription_rules() {
         .await
         .unwrap();
     first
-        .create_channel(
+        .post(
             child,
-            CreateChannelRequest {
-                channel_name: "proofs".into(),
-                subscription: SubscriptionChange::Subscribe,
+            PostRequest {
+                request_id: "create-proofs".into(),
+                destination: PostDestination::NewChannel("proofs".into()),
+                text: "Share proofs here.".into(),
+                agents_to_notify: Vec::new(),
             },
         )
         .await
@@ -115,7 +117,8 @@ async fn shared_handles_resume_posts_and_preserve_subscription_rules() {
         request_id: "call-1".into(),
         destination: PostDestination::Channel("proofs".into()),
         text: "é🦀 proof".into(),
-        agents_to_notify: vec![AgentPath::root(), child_path.clone(), child_path.clone()],
+        // The creator receives this through its channel subscription.
+        agents_to_notify: vec![AgentPath::root()],
     };
     let second = LocalAgentMessageBoard::open(&sqlite, tree, host.clone())
         .await
@@ -180,6 +183,19 @@ async fn shared_handles_resume_posts_and_preserve_subscription_rules() {
         )
         .await
         .unwrap();
+    // Posting to an existing channel must not re-enable its subscription.
+    resumed
+        .post(
+            child,
+            PostRequest {
+                request_id: "one-off-post".into(),
+                destination: PostDestination::Channel("proofs".into()),
+                text: "One more proof, while staying unsubscribed.".into(),
+                agents_to_notify: Vec::new(),
+            },
+        )
+        .await
+        .unwrap();
     // Posting still succeeds when the subscriber lookup returns an empty array.
     resumed
         .post(