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>

e2e/e2e.yml

@@ -0,0 +1,12 @@
+# activity-core e2e test contract
+# Run via: make e2e REPO=activity-core  (from ~/the-custodian)
+name: activity-core
+compose_file: docker-compose.dev.yml
+health_checks:
+  - name: temporal-ui
+    url: http://localhost:8080
+    timeout: 180   # Temporal + Elasticsearch startup is slow (~2min)
+test_command: >-
+  uv run python e2e/tests/test_full_flow.py
+timeout: 300
+cleanup: always