sx-git Phase 4: worktree — tree materialization, index overlay, status (TDD)

Worktree is a value (path->data dict). tree-from-files/tree-files round-trip
through real tree objects (cid-identical to hand-built trees); index =
{:base tree-cid :staged overlay} in kv with add!/rm!/unstage!/index-tree!;
status = three-way dict diff (HEAD vs index vs worktree) with
staged/unstaged/untracked. 26/26, total 132/132.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

lib/git/scoreboard.json

@@ -2,9 +2,10 @@
   "suites": {
     "object": {"pass": 38, "fail": 0},
     "ref": {"pass": 38, "fail": 0},
-    "dag": {"pass": 30, "fail": 0}
+    "dag": {"pass": 30, "fail": 0},
+    "worktree": {"pass": 26, "fail": 0}
   },
-  "total_pass": 106,
+  "total_pass": 132,
   "total_fail": 0,
-  "total": 106
+  "total": 132
 }