Implement issue #15: distinguish draft/complete proposal states with
graded validation messages. fix #15

- Add ChangeStage enum (draft/specified/designed/full) and stage
  detection logic to validation
- Enforce design.md→tasks.md dependency: tasks.md without design.md
  is always an ERROR
- Add graded completeness messages (INFO in non-strict, WARN in strict)
- Add stage column to `llman sdd list` (text and JSON output)
- Add i18n keys for stage hints and constraint error
- Add 3 integration tests covering stage detection and constraint
- Fix 3 pre-existing test fixtures (add design.md + mark tasks done)
- Bump version to 0.0.40

Co-authored-by: Cursor <cursoragent@cursor.com>