feat(reviewer): add Tier 1 reviewer agent role with auto-channel notifications

Reviewer workflow in agent-prompts.ts (8-step process), Rust auto-post
to #review-queue on task->review transition, reviewQueueDepth in
attention scoring (10pts/task cap 50), Tasks tab for reviewer in
ProjectBox with 10s queue polling. 7 vitest + 4 cargo tests.

v2/src-tauri/src/commands/bttask.rs

@@ -36,3 +36,8 @@ pub fn bttask_create(
 pub fn bttask_delete(task_id: String) -> Result<(), String> {
     bttask::delete_task(&task_id)
 }
+
+#[tauri::command]
+pub fn bttask_review_queue_count(group_id: String) -> Result<i64, String> {
+    bttask::review_queue_count(&group_id)
+}