Remove a stray .deb committed by an over-broad git add -A

An audit agent downloaded findutils to check GNU find's operand parsing;
a76f2c0 swept the 298 KB artifact into the tree along with the capability
changes, unmentioned in its message. Second time this session that
`git add -A` has staged something unrelated — ignore the pattern so it
cannot happen a third time.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01GBq2rGum6GX7xXgsas1fDc

.gitignore

@@ -10,3 +10,6 @@ code-review.md
 
 # Local bug-report screenshots, same spirit as Screenshot*.png above.
 screenshot_for_fix/
+
+# Package files pulled in by ad-hoc verification runs.
+*.deb