Restore old system prompt behavior around subagents (#7576)

Author: jamadeo

crates/goose/src/agents/extension_manager.rs

@@ -488,6 +488,7 @@ impl ExtensionManager {
             context: PlatformExtensionContext {
                 extension_manager: None,
                 session_manager,
+                session: None,
             },
             provider,
             tools_cache: Mutex::new(None),
@@ -715,6 +716,11 @@ impl ExtensionManager {
                     })?;
                 let mut context = self.context.clone();
                 context.extension_manager = Some(Arc::downgrade(self));
+                if let Some(id) = session_id {
+                    if let Ok(session) = self.context.session_manager.get_session(id, false).await {
+                        context.session = Some(Arc::new(session));
+                    }
+                }
 
                 (def.client_factory)(context)
             }

crates/goose/src/agents/platform_extensions/mod.rs

@@ -9,6 +9,7 @@ pub mod tom;
 use std::collections::HashMap;
 
 use crate::agents::mcp_client::McpClientTrait;
+use crate::session::Session;
 use once_cell::sync::Lazy;
 
 pub use ext_manager::MANAGE_EXTENSIONS_TOOL_NAME_COMPLETE;
@@ -124,6 +125,7 @@ pub struct PlatformExtensionContext {
     pub extension_manager:
         Option<std::sync::Weak<crate::agents::extension_manager::ExtensionManager>>,
     pub session_manager: std::sync::Arc<crate::session::SessionManager>,
+    pub session: Option<std::sync::Arc<Session>>,
 }
 
 impl PlatformExtensionContext {