feat(e2e): add e2e contract and test script (closes T21)

CUST-WP-0028-T03/T04:
- e2e/e2e.yml: declares stack (docker-compose.dev.yml), Temporal UI
  health check, test command
- e2e/tests/test_full_flow.py: automates WP-0001 T21 — seeds DB, starts
  workers, triggers RunActivityWorkflow, polls completion, asserts
  ActivityRun + TaskInstances written to DB

Run via: make e2e REPO=activity-core  (from ~/the-custodian)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

workplans/custodian-WP-0001-temporal-backbone.md

@@ -88,7 +88,7 @@ tasks:
     state_hub_task_id: 1da921f5-86a8-488f-a015-402079194e10
   - id: T21
     title: Manual end-to-end test
-    status: todo
+    status: done
     state_hub_task_id: f72bba1a-eb24-496e-9498-f4676facc5c9
 ---