feat: rebuild, list, rollback — Phase 5 completion

- kappa rebuild: diffs config vs installed DB, reports changed/added/removed
  packages, detects kernel/init/bootloader/service changes
- kappa list: shows installed packages with versions, hashes, provides
- kappa rollback: lists available generations from /kappa/db/generations
- list + rollback don't require a file argument (no-file subcommands)
- Rebuild wired to compute_changes() → per-package change detection

Full pipeline: resolve → fetch → build → install → list/rollback/rebuild

CMakeLists.txt

@@ -36,5 +36,6 @@ add_executable(kappa
   src/build/build.cpp
   src/sched/scheduler.cpp
   src/install/install.cpp
+  src/rebuild/rebuild.cpp
 )
 target_include_directories(kappa PRIVATE include)