mainline onboarding

Use this skill when someone is new to this repository and needs a reliable first pass through setup, baseline checks, and terminology.

Use this before the worktree skill when the user is still figuring out what this repo is, how it is installed, or which commands are supposed to work in this checkout.

Goal

Separate these two jobs immediately:

• developing the mainline codebase in this repo
• using mainline to manage some other target repo

Do not let the user conflate them.

Read first