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/cmd/convoy.go

@@ -23,7 +23,7 @@ import (
 // generateShortID generates a short random ID (5 lowercase chars).
 func generateShortID() string {
 	b := make([]byte, 3)
-	rand.Read(b)
+	_, _ = rand.Read(b)
 	return strings.ToLower(base32.StdEncoding.EncodeToString(b)[:5])
 }