fix: Address golangci-lint errors (errcheck, gosec) (#76)

Apply PR #76 from dannomayernotabot:

- Add golangci exclusions for internal package false positives
- Tighten file permissions (0644 -> 0600) for sensitive files
- Add ReadHeaderTimeout to HTTP server (slowloris prevention)
- Explicit error ignoring with _ = for intentional cases
- Add //nolint comments with justifications
- Spelling: cancelled -> canceled (US locale)

Co-Authored-By: dannomayernotabot <noreply@github.com>

🤖 Generated with Claude Code

internal/mrqueue/events.go

@@ -78,7 +78,7 @@ func (l *EventLogger) LogEvent(event Event) error {
 	}
 
 	// Append to log file
-	f, err := os.OpenFile(l.logPath, os.O_APPEND|os.O_CREATE|os.O_WRONLY, 0644)
+	f, err := os.OpenFile(l.logPath, os.O_APPEND|os.O_CREATE|os.O_WRONLY, 0600)
 	if err != nil {
 		return fmt.Errorf("opening event log: %w", err)
 	}