feat: add platform-neutral LINE onboarding to setup wizard

[ijbhxhu]

Description

Add a platform-neutral LINE path to the existing openab setup wizard.

The LINE flow should:

• securely prompt for the channel secret and channel access token
• validate the access token through LINE's bot-info API before writing files
• generate a config.toml with explicit [line] trust settings and no credentials
• generate a separate credential artifact for Local/Docker or Kubernetes
• print ready-to-run Local, Docker, and Helm/Kubernetes commands
• optionally register and test a public HTTPS LINE webhook
• use the existing Helm chart's existingSecret path so secrets do not enter Helm values

This should live in the core setup wizard, not in oabctl.

Non-goals

• AWS ECS or oabctl integration
• provisioning ingress, DNS, TLS, or a Kubernetes cluster
• hosted secret custody or web onboarding
• LINE Quick Reply, Flex Message, or Postback-to-agent behavior

Use Case

A user with a LINE Messaging API channel should be able to run:

openab setup --platform line

and receive deployment-ready configuration and secret artifacts without manually editing config.toml or learning cloud-specific infrastructure. The same onboarding contract should work for local development, Docker, and Kubernetes.