feat(app): config migrate facade + init 주석 공유 + doctor 체크

- config_migrate_with_config_path: 백업(.bak)+atomic write(tmp→rename)+dry-run,
  round-trip 검증으로 실패 시 원본 보존. ConfigMigrationReport 반환.
- init_workspace 가 annotated_default_document() 사용(섹션 주석 포함).
- doctor 에 config_migration 체크 추가(미동기 시 ok=false + hint).
- tests/config_migrate.rs 4개(백업/atomic/dry-run/멱등/doctor) 통과.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

crates/kebab-app/Cargo.toml

@@ -71,6 +71,7 @@ base64               = { workspace = true }
 rusqlite             = { workspace = true }
 
 [dev-dependencies]
+kebab-config = { path = "../kebab-config" }
 # doc-side expansion (Phase 2) Task 4: ExpansionGenerator unit tests build
 # MockLanguageModel (gated behind kebab-llm's `mock` feature, default OFF in
 # [dependencies]). Enabling it here turns it on for the test build only.