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.md

@@ -7,4 +7,5 @@ _Generated by `lib/git/conformance.sh`_
 | object | 38 | 0 | 38 |
 | ref | 38 | 0 | 38 |
 | dag | 30 | 0 | 30 |
-| **Total** | **106** | **0** | **106** |
+| worktree | 26 | 0 | 26 |
+| **Total** | **132** | **0** | **132** |