[init] impr: add agent flags to neon init

[rodneyshibu]

Context
The init command serves to install Neon's VS Code/ VSX extension or Neon's MCP server. We want to support the ability to add flags to specify which agent the user wants the tools installed for.

What changes are proposed in this pull request?
This PR makes two changes

1. We add a --agent flag where the user can specify their agent to set up with.
2. We unset CI before executing auth command. This ensures that agent chats can open the auth URL in the browser.

How was this tested?
Copilot

Cursor

Claude

#LKB-9661