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/event.rs

@@ -33,4 +33,18 @@ pub enum SubagentEvent {
         output: String,
     },
     Progress(String),
+    /// Token usage reported by the LLM after one streaming call inside the
+    /// subagent. The drain thread accumulates these across all steps and
+    /// forwards the total to the parent's `TurnEvent::ReviewUsage` handler
+    /// so the Usage panel can split "main" tokens from "self-learning"
+    /// tokens (review, test-gen, arch-review, security-review, etc.).
+    ///
+    /// Why a separate variant instead of folding into `Completed`: usage
+    /// is reported per-step, so the parent can update the running total
+    /// incrementally rather than waiting for the whole subagent run to
+    /// finish. The drain thread still aggregates before forwarding.
+    Usage {
+        tokens_in: u64,
+        tokens_out: u64,
+    },
 }