Rewrite the README for first-time readers and add a getting-started guide

The README is rewritten for GitLab team members meeting the project for the first time.

• It opens with one plain sentence, then two on why you would use it.
• Six small diagrams show the overview, the contribute and reproduce paths, a "which path do I need" chooser, who approves what, and the repository map.
• A glossary defines rig, base, lane, arm and the other terms once.
• Install is two lines, then a link to the new guide.

docs/getting-started.md covers prerequisites, a first contribution, a first simulation (with cost and auto-delete) and troubleshooting with just lab doctor. Each walkthrough has one sequence diagram. docs/README.md indexes it.

Every diagram was render-checked. just ci passes.

Glance test

The maintenance guide now records a README glance test: a fresh agent session gets only README.md and answers five questions. The first rewrite scored 3 of 5. It had no example ask, and "rig", "lane" and "vendored skill" read as jargon. A second commit fixed those. The rerun scored 4 of 5, named both paths and the plain-words ask, and named both waits: your yes before posting, and your approval before a VM costs money.