feat(adapters): GitShardAdapter history adopt + cross-substrate integration (WP-0012 T3)

Adopt git-native history (TSD §A.5): a VERSION-gated history(key) surfaces the
commit list for a path (newest-first sha + subject) — declared by every git-IS-store
shard, read-only or not. Integration proves the union/overlay/edit machinery works
unchanged across folder + git substrates: resolve/chorus span both, edit through a
git shard fast-forwards as a commit, apply-under-drift refuses on an external commit
(sha drift) without clobbering, and a read-only git target keeps the overlay as a
draft. SCOPE updated; WP-0012 done. 196 tests green.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/shard_wiki/adapters/__init__.py

@@ -9,12 +9,13 @@ from shard_wiki.adapters.conformance import (
 )
 from shard_wiki.adapters.contract import CONTRACT_VERSION, ShardAdapter
 from shard_wiki.adapters.folder import FolderAdapter
-from shard_wiki.adapters.git import GitShardAdapter
+from shard_wiki.adapters.git import GitShardAdapter, PageRevision
 
 __all__ = [
     "ShardAdapter",
     "FolderAdapter",
     "GitShardAdapter",
+    "PageRevision",
     "CONTRACT_VERSION",
     "Check",
     "ConformanceReport",