fix: apply review fixes, update version to v0.20, add Sprint 18 changelog

- Fix stale tree cache: clear _dirCache and _expandedDirs on root nav
- Fix clearPreview: prompt before discarding unsaved preview edits
- Update UI version label from v0.17.1 to v0.20
- Add Sprint 18 entry to CHANGELOG.md
- Update SPRINTS.md current state to v0.20

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

SPRINTS.md

@@ -1,6 +1,6 @@
 # Hermes Web UI -- Forward Sprint Plan
 
-> Current state: v0.19 | 318 tests | Daily driver ready
+> Current state: v0.20 | 318 tests | Daily driver ready
 > This document plans the path from here to two targets:
 >
 > Target A: 1:1 feature parity with the Hermes CLI (everything you can do from the