mainline

Work in a dedicated Git worktree, make all commits there, and land changes through `mq` instead of merging directly into `main`. Use after onboarding, once the user already understands the repo's setup and wants the contribution workflow.

First skill for new contributors, fresh clones, and setup questions in the mainline repo. Separates developing mainline from using mq on another repo, verifies the local baseline, explains canonical-root trust, and clarifies which commands should or should not work in this source checkout.