Use shared runtime logs bucket

[Pigbibi]

Summary

• update snapshot input/artifact workflow defaults to use gs://qsl-runtime-logs-shared
• update strategy plugin artifact defaults and snapshot health defaults
• update runbook/contract docs and workflow tests

Verification

• migrated shared bucket objects and verified counts match: total 2486, execution reports 2141, strategy artifacts 345
• no repository variables reference the old bucket
• python -m pytest tests/test_strategy_plugin_publish_workflow.py -q
• git diff --check