Remove stray .cs.bak files and ignore *.bak

Three editor backup files were accidentally committed under
src/CalendarMcp.Core/Tools/. Delete them and add *.bak to .gitignore
so they don't get re-added.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: rockfordlhotka

.gitignore

@@ -494,3 +494,6 @@ $RECYCLE.BIN/
 # Vim temporary swap files
 *.swp
 .claude/settings.local.json
+
+# Editor/backup files
+*.bak