chore(gitignore): ignore *.backup and *.bak scratch files

Keeps stale backup files (e.g. an in-flight
`testing-and-benchmarking/TESTING-TAXONOMY.adoc.backup` snapshot
left by an editor) from showing up as untracked noise.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: hyperpolymath

.gitignore

@@ -101,3 +101,7 @@ k9-coordination-protocol/tools/k9-init/Cargo.lock
 hooks/playbook-to-recipe/Cargo.lock
 .verisimdb/ecosystem-ingest/target/
 .verisimdb/ecosystem-ingest/Cargo.lock
+
+# Backup/scratch files (never commit)
+*.backup
+*.bak