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>

workplans/SHARD-WP-0012-git-shard-adapter.md

@@ -4,7 +4,7 @@ type: workplan
 title: "second adapter — git-IS-store shard (contract validation on a new substrate)"
 domain: whynot
 repo: shard-wiki
-status: active
+status: done
 owner: tegwick
 topic_slug: whynot
 created: "2026-06-15"
@@ -40,7 +40,7 @@ merge beyond fast-forward (apply-under-drift refuse is enough, as in SHARD-WP-00
 
 ```task
 id: SHARD-WP-0012-T1
-status: todo
+status: done
 priority: high
 state_hub_task_id: "8a1c7c80-a0cc-4e02-a611-1f1fd7dec57b"
 ```
@@ -54,7 +54,7 @@ implication rules. Tests: read tracked files; profile validates; conformance rea
 
 ```task
 id: SHARD-WP-0012-T2
-status: todo
+status: done
 priority: high
 state_hub_task_id: "b47dfb86-46c1-4e97-a62f-377719499ff2"
 ```
@@ -68,7 +68,7 @@ changes after an external commit.
 
 ```task
 id: SHARD-WP-0012-T3
-status: todo
+status: done
 priority: medium
 state_hub_task_id: "4c895f42-671d-4948-8bdf-941fd85644bb"
 ```