Phase #2 for #3896.

Scope:

• Migrate messaging channel enrollment for Telegram, Discord, WeChat, Slack, and WhatsApp to the channel manifest and workflow compiler architecture.
• Preserve the existing onboard UX while letting enrollment hooks own channel-specific prompts and setup behavior.
• Keep channel availability and setup driven by ChannelManifestAvailabilityContext and compiled manifest plans.

Validation:

• CLI typecheck/build and targeted onboarding/messaging tests.