feat: directory moves — mirrored batch relocation of every indexed file under a dir

- MovePlan gains moves[] (file move = 1 relocation), left_behind[] for
  unindexable files that deliberately stay, prune_dirs[] applied after the
  batch (remove_dir only succeeds when empty, leftovers keep their home)
- plan::dir: nesting/target-exists guards, merged per-file rewrites
- apply: atomic N-rename batch (per-file git mv), rollback restores every
  move, rewrite and created dir newest-first
- mv_reject accepts dirs; --json adds moved_files/would_move_files only for
  real dir moves (single-file contract byte-identical); summary shows
  (N files) and left-behind count
- engine rollback/prune tests moved to tests/apply.rs (public API; a
  mid-batch failure cannot be forced through the CLI), java-rule e2e split
  into its own binary to keep every file under the 250-line budget

README.md

@@ -57,6 +57,9 @@ jmove mv src/utils/parser.ts src/core/parser.ts
 # --no-git forces a plain rename
 jmove mv src/foo.ts src/bar/foo.ts --no-git
 
+# Move a whole directory: every file relocates, every importer follows
+jmove mv src/utils src/helpers
+
 # Java: jmove updates `package`, all `import`s and moves the file
 jmove mv src/com/example/utils/Parser.java src/com/example/core/Parser.java