feat(KB-041): add task refine capability across core, API, UI, and CLI

- Add refineTask() to TaskStore with validation for feedback and requirements
- Add POST /tasks/:id/refine API endpoint with tests
- Add refine UI to TaskDetailModal for submitting feedback
- Add kb_task_refine tool to pi extension for AI-driven refinement
- Add 'kb task refine' CLI command for manual task refinement
- Include validation constraints: max 2000 chars feedback, max 5000 chars requirements

packages/cli/src/__tests__/extension.test.ts

@@ -81,6 +81,7 @@ describe("kb pi extension", () => {
         "kb_task_pause",
         "kb_task_unpause",
         "kb_task_duplicate",
+        "kb_task_refine",
         "kb_task_import_github",
         "kb_task_import_github_issue",
         "kb_task_browse_github_issues",