feat: Implement lesson generation feature and update status display

- Added functionality to generate lessons based on code reviews, including prompts and instructions for the reviewer.
- Updated the `.gitignore` to exclude lesson documentation files.
- Removed the `/lesson` command from the help menu and command parsing.
- Enhanced the status bar to display a message when a lesson is being generated.
- Introduced a new `lesson_running` state to track lesson generation progress.
- Updated various components to handle the new lesson generation workflow, including subagent events and token usage tracking.

src/app/subagent/engine.rs

@@ -383,6 +383,7 @@ pub fn run_subagent(ctx: &SubagentContext, tx: &mpsc::Sender<SubagentEvent>) ->
         let tx_clone = tx.clone();
         let mut current_thinking = String::new();
         let mut current_token = String::new();
+        let mut step_usage: Option<(u64, u64)> = None;
 
         // Use streaming API so the abort flag is checked per SSE event,
         // making the subagent responsive to cancellation even during an
@@ -408,13 +409,20 @@ pub fn run_subagent(ctx: &SubagentContext, tx: &mpsc::Sender<SubagentEvent>) ->
                         let prog = format_subagent_progress("replying", &current_token);
                         let _ = tx_clone.blocking_send(SubagentEvent::Progress(prog));
                     }
+                    crate::app::runtime::stream::StreamEvent::Usage { prompt_tokens, completion_tokens, .. } => {
+                        // Capture usage so the drain thread can route it
+                        // to the parent's `UsageStats::review_tokens`.
+                        // Last writer wins — providers send exactly one
+                        // Usage event per streaming call.
+                        step_usage = Some((*prompt_tokens, *completion_tokens));
+                    }
                     _ => {}
                 }
                 true
             },
         );
 
-        let (response, _usage) = match stream_result {
+        let (response, returned_usage) = match stream_result {
             Ok(result) => result,
             Err(e) => {
                 let is_abort = ctx.abort_flag.as_ref().is_some_and(|f| f.load(std::sync::atomic::Ordering::SeqCst))
@@ -437,6 +445,17 @@ pub fn run_subagent(ctx: &SubagentContext, tx: &mpsc::Sender<SubagentEvent>) ->
             }
         };
 
+        // Emit the token usage from this streaming call so the parent's
+        // drain thread can accumulate it and update the Usage panel.
+        // Without this, the Usage panel always shows zeros because the
+        // subagent never tells the parent about the tokens consumed.
+        if let Some((tokens_in, tokens_out)) = returned_usage {
+            let _ = tx.blocking_send(SubagentEvent::Usage {
+                tokens_in,
+                tokens_out,
+            });
+        }
+
         let has_tool_calls = response.tool_calls.is_some()
             && response.tool_calls.as_ref().is_some_and(|tc| !tc.is_empty());