feat(KB-181): add kb task plan command with planning mode

- Add kb task plan CLI command for interactive planning mode
- Create planTask tool in extension for LLM-initiated planning sessions
- Add comprehensive tests for task plan command with edge cases
- Update QuickEntryBox UI with planning mode button
- Add changeset for planning mode CLI feature

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

@@ -87,6 +87,7 @@ describe("kb pi extension", () => {
         "kb_task_browse_github_issues",
         "kb_task_archive",
         "kb_task_unarchive",
+        "kb_task_plan",
       ];
 
       for (const name of expected) {