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/conformance.sh

@@ -13,7 +13,7 @@ if [ ! -x "$SX_SERVER" ]; then
   exit 1
 fi
 
-SUITES=(object ref dag worktree diff merge)
+SUITES=(object ref dag worktree diff merge porcelain)
 
 OUT_JSON="lib/git/scoreboard.json"
 OUT_MD="lib/git/scoreboard.md"
@@ -48,6 +48,7 @@ run_suite() {
 (load "lib/git/worktree.sx")
 (load "lib/git/diff.sx")
 (load "lib/git/merge.sx")
+(load "lib/git/porcelain.sx")
 (epoch 2)
 (eval "(define git-test-pass 0)")
 (eval "(define git-test-fail 0)")