Add activity-core LLM endpoint support

fixtures/activity_core/README.md

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+# Activity-Core Daily Triage Fixture
+
+These non-secret fixtures mirror the `daily-triage-report` instruction in the
+activity-core Railiance runtime as reviewed on 2026-06-07.
+
+Source context:
+
+- `/home/worsch/activity-core/k8s/railiance/20-runtime.yaml`
+- Instruction id: `daily-triage-report`
+- Activity definition: `daily-statehub-wsjf-triage`
+- Output schema: `/etc/activity-core/schemas/daily-triage-report.json`
+
+The execute request fixture contains only dummy digest data. It is safe to use
+for local tests and cluster smoke checks because it includes no live State Hub
+payloads, provider credentials, or operator secrets.