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

docs/SKILL.md

@@ -2,8 +2,8 @@
 
 ## What this tool does
 
-Moves or renames source files inside a project and updates every import
-statement referencing them. Never breaks imports. Supported: TypeScript,
+Moves or renames source files — or whole directories of them — inside a
+project and updates every import statement referencing them. Never breaks imports. Supported: TypeScript,
 JavaScript, Java (package declaration + all importers + the file move are
 kept in sync); Python/Go on the roadmap. Single binary, no LSP needed.
 
@@ -25,6 +25,13 @@ Always run `--dry-run` first and confirm the change set looks right.
 Moving onto an existing path fails with `TARGET_EXISTS` — choose another
 target (Phase 1 has no overwrite mode).
 
+Directory moves: `mv <dir> <target>` relocates every indexed file under
+`<dir>` mirrored under `<target>` in one atomic batch. `--json` adds
+`moved_files[]` (from/to per file) and `left_behind[]` — real files under
+`<dir>` that are not indexable and deliberately stay where they are.
+Emptied source directories are pruned; a directory with leftovers is not.
+A source that is nested in its own target fails with `PLAN_REJECTED`.
+
 Git integration: inside a git repository, a tracked file is renamed with
 `git mv` so the rename is staged (history-preserving `git log --follow` /
 `git diff -M` work). Untracked files, non-repositories and `--no-git`