sx-git Phase 7: porcelain — init/add/commit/branch/checkout/tag/reset/merge/log/diff (TDD)

End-to-end topology story: fork, diverge, real merge commit (parents in
order), fast-forward + up-to-date, annotated + lightweight tags, soft/mixed
reset, detached HEAD commits, staged/unstaged unified diffs. Conflicted
merges park MERGE_HEAD + stage the marker tree; git/merge-commit! concludes
with two parents after resolution. Extensible commit meta flows through
porcelain (agentic-sx shape verified). 40/40 — ROADMAP COMPLETE 227/227.

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

lib/git/scoreboard.json

@@ -5,9 +5,10 @@
     "dag": {"pass": 30, "fail": 0},
     "worktree": {"pass": 26, "fail": 0},
     "diff": {"pass": 27, "fail": 0},
-    "merge": {"pass": 28, "fail": 0}
+    "merge": {"pass": 28, "fail": 0},
+    "porcelain": {"pass": 40, "fail": 0}
   },
-  "total_pass": 187,
+  "total_pass": 227,
   "total_fail": 0,
-  "total": 187
+  "total": 227
 }