Add real-time research completion notifications

When background research completes, g3 now immediately prints a status
message instead of waiting for the next user interaction:

- Added ResearchCompletionNotification and broadcast channel to
  PendingResearchManager for push-based notifications
- Added spawn_research_notification_handler() in interactive mode that
  listens for completions in a background task
- When idle (at prompt): clears line, prints status, reprints prompt
- When busy (processing): prints status inline (interleaving is fine)
- Added G3Status::research_complete() for consistent formatting
- Added enable_research_notifications() method to Agent

Output format: "g3: 1 research report ... [done]"

crates/g3-core/src/lib.rs

@@ -38,6 +38,9 @@ pub use task_result::TaskResult;
 // Re-export context window types
 pub use context_window::{ContextWindow, ThinResult, ThinScope};
 
+// Re-export pending research types for notification handling
+pub use pending_research::{PendingResearchManager, ResearchCompletionNotification, ResearchStatus};
+
 // Export agent prompt generation for CLI use
 pub use prompts::get_agent_system_prompt;
 
@@ -1484,6 +1487,26 @@ impl<W: UiWriter> Agent<W> {
         &self.pending_research_manager
     }
 
+    /// Subscribe to research completion notifications.
+    ///
+    /// Returns a receiver that will receive notifications when research tasks complete.
+    /// Returns None if the agent was not configured with notifications enabled.
+    /// Use this in interactive mode to get real-time updates when research finishes.
+    pub fn subscribe_research_notifications(&self) -> Option<tokio::sync::broadcast::Receiver<pending_research::ResearchCompletionNotification>> {
+        self.pending_research_manager.subscribe()
+    }
+
+    /// Enable research completion notifications and return a receiver.
+    ///
+    /// This replaces the internal research manager with one that sends notifications.
+    /// Call this once during setup (e.g., in interactive mode) before any research tasks.
+    /// Returns a receiver that will receive notifications when research tasks complete.
+    pub fn enable_research_notifications(&mut self) -> tokio::sync::broadcast::Receiver<pending_research::ResearchCompletionNotification> {
+        let (manager, rx) = pending_research::PendingResearchManager::with_notifications();
+        self.pending_research_manager = manager;
+        rx
+    }
+
     pub fn set_requirements_sha(&mut self, sha: String) {
         self.requirements_sha = Some(sha);
     }