docs: v0.26 release notes, remove planning artifact, update versions

- Add v0.26 CHANGELOG entry (10 post-Sprint-23 fixes)
- Remove SPRINT_23_PLAN.md (planning artifact, not runtime docs)
- Bump version label to v0.26 in index.html
- Update SPRINTS header and footer to v0.26 / 426 tests
- Update CHANGELOG footer to v0.26 / 426 tests

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: v0.25 | 423 tests | Daily driver ready
+> Current state: v0.26 | 426 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
@@ -663,5 +663,5 @@ and switchToProfile() didn't refresh workspaces or sessions.
 ---
 
 *Last updated: April 3, 2026*
-*Current version: v0.25 | 423 tests*
+*Current version: v0.26 | 426 tests*
 *Next sprint: Sprint 24 (Desktop Application)*