[repo-assist] fix: clean up ALPM lock on interrupt

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Closes #2918

Root cause

The process used context.Background() while relying on deferred dbExecutor.Cleanup(). Go terminates immediately on the default SIGINT path, so Ctrl+C bypassed deferred cleanup and could leave /var/lib/pacman/db.lck behind.

Fix

• Use signal.NotifyContext for SIGINT at process entry.
• Let cancellation propagate through the existing context-aware operations so normal deferred cleanup runs.
• Add a regression test that verifies an interrupt cancels the application context.

Test Status

• go test . -count=1 — passed
• make build — passed
• make lint — not run successfully because golangci-lint is unavailable in the environment

No new dependencies were added.

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