feat(adapters): GitShardAdapter read path + git-IS-store profile (WP-0012 T1)

A second substrate validating the contract beyond plain folders: a git-IS-store
shard reading Markdown from a git repo. Keys are tracked *.md paths; read returns
a Page whose source_rev is the per-path last-commit sha (so an edit to one page
never drifts another); profile is git-IS-store / substrate=git / history=git-native
/ addressing=path, validated against the §6.5 implication rules. Passes the
conformance read path with honest absence of unclaimed verbs. Zero new deps
(git CLI via subprocess). No core changes.

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

src/shard_wiki/adapters/__init__.py

@@ -9,10 +9,12 @@ 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
 
 __all__ = [
     "ShardAdapter",
     "FolderAdapter",
+    "GitShardAdapter",
     "CONTRACT_VERSION",
     "Check",
     "ConformanceReport",