Remove stale SONNET-TASKS.md and add to .gitignore

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

Author: hyperpolymath

.gitignore

@@ -117,4 +117,4 @@ htmlcov/
 # Temp
 /tmp/
 *.tmp
-*.bak
+*.bakSONNET-TASKS.md