feat(KB-636): add Missions system for large-scale project planning

- Fix TaskStore sliceId persistence and bidirectional linking between tasks and features
- Add comprehensive mission integration tests for hierarchy, status rollup, and events
- Enhance E2E tests with reorder, cascade delete, and error handling coverage
- Add scheduler integration tests for mission slice activation
- Fix duplicate mission commands in CLI help text
- Add changeset for missions launch

.changeset/missions-launch.md

@@ -13,11 +13,14 @@ The Missions system provides a hierarchical planning structure:
 **New Features:**
 - SQLite database schema for mission hierarchy with automatic status rollup
 - MissionStore with full CRUD operations and event emissions
-- REST API endpoints for mission, milestone, slice, and feature management
+- REST API with AI-driven interview system for interactive planning
+- Dashboard UI: mission list, hierarchical detail view, timeline visualization
 - CLI commands: `fn mission create`, `list`, `show`, `delete`, `activate-slice`
-- Dashboard UI components for mission management
+- Pi extension tools for chat-based mission management
+- Engine integration: automatic slice activation when linked tasks complete
 
 **Usage:**
-- Create missions with `fn mission create "Title" "Description"`
-- View hierarchy with `fn mission show <id>`
+- Press Cmd/Ctrl+Shift+M in dashboard to open missions
+- Use interview mode for AI-assisted mission planning
 - Link features to tasks for automatic progress tracking
+