Fix tool result counting on session restart

[Tarquinen]

Summary

• Fix tool result counting to not re-count already-pruned tools when a session is restarted
• Properly clear tool tracker state (seenToolResultIds, skipNextIdle) when switching sessions

Details

When restarting a session, the tool cache sync was counting all tool results again, including those that had already been pruned. This could cause the pruning threshold to be reached prematurely.

This fix:

1. Adds a clearToolTracker function that resets all tracker state when switching sessions
2. Modifies syncToolCache to skip already-pruned tools when counting toward the nudge threshold
3. Removes unused PruneReason import from janitor.ts

Version bump: 0.4.15 → 0.4.16