Add paginated channel subscription queries to the local message board (#56495)

## What changed

Add `LocalAgentMessageBoard::subscribed_channels` to list the caller's subscribed channel names on the current board in name order, using the existing `PageRequest` limits and cursors.

## Testing

Extend the local board test to verify that a read-only connection can retrieve two subscribed channels across successive one-item pages, with no cursor after the final page.

GitOrigin-RevId: 63f2e3e7caad9638e5e0f4f38aaf722a590c5d7f

codex-rs/message-board/src/local/queries.rs

@@ -19,6 +19,7 @@ use crate::ChannelQuery;
 use crate::ChannelSummary;
 use crate::CreateChannelRequest;
 use crate::Page;
+use crate::PageRequest;
 use crate::PostContent;
 use crate::PostMetadata;
 use crate::PostPreview;
@@ -80,6 +81,31 @@ impl LocalAgentMessageBoard {
             .execute(&mut *tx).await.map_err(storage_error)?;
         tx.commit().await.map_err(storage_error)
     }
+
+    /// Lists the caller's channel subscriptions within the existing page budget.
+    pub async fn subscribed_channels(
+        &self,
+        caller: ThreadId,
+        page: PageRequest,
+    ) -> Result<Page<String>> {
+        self.host.agent_path(caller).await?;
+        let window = Window::new(&page)?;
+        // Probe the existing subscriptions primary key for each channel.
+        let channels = sqlx::query_scalar::<_, String>(
+            "SELECT c.name FROM channels AS c WHERE c.board=? AND EXISTS (
+                SELECT 1 FROM subscriptions AS s
+                WHERE s.board=c.board AND s.target=json_object('Channel',c.name) AND s.agent=?
+             ) ORDER BY c.name LIMIT ? OFFSET ?",
+        )
+        .bind(self.identity.to_string())
+        .bind(caller.to_string())
+        .bind((window.limit + 1) as i64)
+        .bind(window.offset())
+        .fetch_all(self.pool.as_ref())
+        .await
+        .map_err(storage_error)?;
+        window.finish(channels)
+    }
 }
 
 impl AgentMessageBoard for LocalAgentMessageBoard {

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

@@ -1936,6 +1936,34 @@ async fn silent_appends_preserve_evidence_and_explicit_subscriptions()
             .await?,
         0
     );
+    let first = reader
+        .subscribed_channels(
+            child,
+            PageRequest {
+                cursor: None,
+                limit: NonZeroU32::MIN,
+            },
+        )
+        .await?;
+    let last = reader
+        .subscribed_channels(
+            child,
+            PageRequest {
+                cursor: first.next_cursor,
+                limit: NonZeroU32::MIN,
+            },
+        )
+        .await?;
+    assert_eq!(
+        (first.results, last),
+        (
+            vec![channel.into()],
+            Page {
+                results: vec!["release".into()],
+                next_cursor: None,
+            }
+        )
+    );
     // Normal posts on this same board still subscribe their author and notify once.
     let discussion = PostRequest {
         request_id: "discussion".into(),