fix: port main branch changes to channel subpackages after rebase

Port changes that were applied to the old pkg/channels/*.go files on main
to their new locations in channel subpackages:
- telegram: precompile regex, var transcribedText, GetModelName()
- discord: var transcribedText declaration
- onebot: resp.Body.Close(), "canceled" spelling, remove empty line
- slack: named return values in parseSlackChatID
- wecom: remove sendMarkdownMessage dead code
- whatsapp: resp.Body.Close() after Dial
- gateway/helpers: remove unused errors import

cmd/picoclaw/internal/gateway/helpers.go

@@ -176,7 +176,6 @@ func gatewayCmd(debug bool) error {
 		fmt.Printf("Error starting channels: %v\n", err)
 	}
 
-
 	fmt.Printf("✓ Health endpoints available at http://%s:%d/health and /ready\n", cfg.Gateway.Host, cfg.Gateway.Port)
 
 	go agentLoop.Run(ctx)