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/mail.go

@@ -433,7 +433,7 @@ func init() {
 	// Reply flags
 	mailReplyCmd.Flags().StringVarP(&mailReplySubject, "subject", "s", "", "Override reply subject (default: Re: <original>)")
 	mailReplyCmd.Flags().StringVarP(&mailReplyMessage, "message", "m", "", "Reply message body (required)")
-	mailReplyCmd.MarkFlagRequired("message")
+	_ = mailReplyCmd.MarkFlagRequired("message")
 
 	// Search flags
 	mailSearchCmd.Flags().StringVar(&mailSearchFrom, "from", "", "Filter by sender address")