fix: remove dead activity tracker summarizers

[tmustier]

Summary

• remove the unused tool arg/result summarizer helpers from activity-tracker.ts
• keep the currently used extractStart*/extractEnd* path unchanged
• clear the last remaining lint warnings in this package

Testing

• npm run check
• npm run smoke-test

[tmustier]

Findings

No issues found. This is a clean dead-code removal — the deleted functions are unreferenced, have no test coverage to break, and their replacements are already wired in and working.

Verdict

correct — pure dead-code deletion with no functional or behavioral change.