MCP upgrade version string sweep must cover entire source tree not just upgrade script targets

GitHub repo UPGRADE_SUMMARY claimed “Zero v17.1 references remaining” but 11 stale v17.1 references and 6 stale 3.0.0 references survived in src/data/ tier files (audit_log.py, redis_cache.py, sqlite_store.py, metrics_store.py, init.py). The upgrade script missed docstring headers in data tier modules. One was a runtime version string in metrics_store.py:165 ("version": "3.0.0") — not just a comment. Fix: sed -i batch replacement on all data tier files. Lesson: Always grep -rn the ENTIRE source tree for old version strings after bulk upgrades, not just the files the upgrade script claims to have changed.