Move agent message board host integration to app-server (#59162)

## What changed

Move the board host adapter and `install_agent_message_board` from `codex-core` to `codex-app-server`, using public core runtime interfaces. Update extension registration, test callers, dependencies, and installation documentation to use the new location.

Format `CHANNEL_POST` notification payloads in the host adapter and retain recognition of legacy board notices in core. Expose `MULTI_AGENT_V2_NAMESPACE_DESCRIPTION` for host extensions to share with native tools.

GitOrigin-RevId: 679b5751de102bff80e6b9d4b0728a84fbdedd85

codex-rs/core/src/context/agent_message_board_notification.rs

@@ -1,12 +1,11 @@
-//! Attributed discussion notices with bounded previews.
+//! Recognizes legacy board notices in saved user-role messages. New notices use agent mail.
 
 use super::ContextualUserFragment;
-use codex_message_board::PostPreview;
 use codex_protocol::models::ContentItem;
 use codex_protocol::models::ContentItemKind;
 use codex_protocol::models::input_text;
 
-pub(crate) struct AgentMessageBoardNotification(pub(crate) PostPreview);
+pub(crate) struct AgentMessageBoardNotification;
 
 impl ContextualUserFragment for AgentMessageBoardNotification {
     fn role(&self) -> &'static str {
@@ -26,19 +25,6 @@ impl ContextualUserFragment for AgentMessageBoardNotification {
         )
     }
     fn body(&self) -> ContentItem {
-        let post = &self.0;
-        let suffix = if post.truncated {
-            "\n[Use read_post for the rest.]"
-        } else {
-            ""
-        };
-        input_text(format!(
-            "Message Type: CHANNEL_POST\nSender: {}\nChannel: {}\nMessage ID: {}\nThread ID: {}\nPayload:\n{}{suffix}",
-            post.metadata.author,
-            post.metadata.channel_name,
-            post.metadata.message_id,
-            post.metadata.thread_id,
-            post.text_preview,
-        ))
+        input_text(String::new())
     }
 }