feat(adapters): ShardAdapter contract + read-only FolderAdapter (WP-0007 T3)

Versioned ShardAdapter ABC (shard_id/profile/keys/read mandatory; optional verbs
raise NotSupported by default = honest absence). FolderAdapter reads a dir of
Markdown into Pages (relpath=key, mtime=source_rev, provenance envelope),
declaring a validated read-only file-store profile. 4 tests green.

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

src/shard_wiki/adapters/__init__.py

@@ -0,0 +1,6 @@
+"""adapters/ — the shard adapter contract (bottom waist) and concrete adapters."""
+
+from shard_wiki.adapters.contract import CONTRACT_VERSION, ShardAdapter
+from shard_wiki.adapters.folder import FolderAdapter
+
+__all__ = ["ShardAdapter", "FolderAdapter", "CONTRACT_VERSION"]