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/model/app_config.rs

@@ -201,12 +201,9 @@ mod tests {
         // In the test runner's environment ANTHROPIC_BASE_URL and
         // ANTHROPIC_API_KEY may or may not be set — we only verify that
         // the function returns Some(..) when both are present.
-        let (b, k) = match claude_credentials_from_env() {
-            Some(v) => v,
-            None => {
-                // Not an error: CI / local without the vars.
-                return;
-            }
+        let Some((b, k)) = claude_credentials_from_env() else {
+            // Not an error: CI / local without the vars.
+            return;
         };
         assert!(!b.is_empty(), "ANTHROPIC_BASE_URL must not be empty");
         assert!(!k.is_empty(), "ANTHROPIC_API_KEY must not be empty");