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

.changeset/add-planning-mode-cli.md

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+---
+"@dustinbyrne/kb": minor
+---
+
+Add `kb task plan` command for interactive AI-guided task planning
+
+The new planning mode guides users through an AI-driven conversation to transform vague ideas into well-specified tasks. The CLI asks clarifying questions, gathers requirements, and generates a structured task summary.
+
+Usage:
+  kb task plan [description] [--yes]
+
+Features:
+- Interactive Q&A with multiple question types (text, single_select, multi_select, confirm)
+- Multi-line text input support (type DONE to finish)
+- Automatic task creation from planning summary
+- --yes flag for non-interactive task creation
+- Rate limit and error handling
+- Extension tool `kb_task_plan` for pi integration