feat(coordination): git backend wiring + verbatim log migration (WP-0009 T4)

InformationSpace.git_backed(space_id, repo_path) wires the git coordination log;
the default constructor stays in-memory for tests (new keyword-only store=). A
one-time importer (migrate_space / import_log / JSONL export+import) replays an
existing in-memory or JSON log into git verbatim — preserving seq, timestamp and
actor (union-without-erasure) and refusing out-of-order import. Same fold after
migration; no behavioural change to overlay/union. SCOPE updated; WP-0009 done.

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

workplans/SHARD-WP-0009-git-decision-log.md

@@ -4,7 +4,7 @@ type: workplan
 title: "git-backed DecisionLog + per-space append authority"
 domain: whynot
 repo: shard-wiki
-status: active
+status: done
 owner: tegwick
 topic_slug: whynot
 created: "2026-06-15"
@@ -39,7 +39,7 @@ sharding (blueprint O-12). Single append authority per space is the target.
 
 ```task
 id: SHARD-WP-0009-T1
-status: todo
+status: done
 priority: high
 state_hub_task_id: "a8fcbb3e-fbc4-4f68-9cf0-d8a6ee057191"
 ```
@@ -54,7 +54,7 @@ ordering preserved; deterministic serialization.
 
 ```task
 id: SHARD-WP-0009-T2
-status: todo
+status: done
 priority: high
 state_hub_task_id: "62abd162-4243-4659-8d27-9fc967ab11a0"
 ```
@@ -69,7 +69,7 @@ hand-off resumes from head; a partitioned non-holder cannot fork the log.
 
 ```task
 id: SHARD-WP-0009-T3
-status: todo
+status: done
 priority: high
 state_hub_task_id: "8cc3691e-05a7-443f-9292-a3fdf3fd59a4"
 ```
@@ -82,7 +82,7 @@ process B (new handle) sees it; fold equals the in-memory fold for the same even
 
 ```task
 id: SHARD-WP-0009-T4
-status: todo
+status: done
 priority: medium
 state_hub_task_id: "281e1db4-6a75-456b-a2bc-b761feb10609"
 ```