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

packages/cli/src/bin.ts

@@ -104,7 +104,7 @@ Usage:
   fn mission create [title] [description]  Create a new mission
   fn mission list                       List all missions
   fn mission show <id>                  Show mission with hierarchy
-  fn mission delete <id> [--force]      Delete a mission
+  fn mission delete <id> [--force]      Delete mission
   fn mission activate-slice <slice-id>  Activate a pending slice
 
 Options: