fix(onboarding+scaffold): setup flow stops overclaiming — chips do what they say, new users see the push snippet #5549

Flow audit #9/#10 + the onboarding copy-honesty batch:

• Starter chips were three labels for one behavior: scaffoldFirstRepo took no starter parameter, so "README + MIT" wrote no LICENSE and "Node + .gitignore" wrote no .gitignore, ever. The scaffold now takes the chip the user clicked and writes LICENSE (MIT with year + holder) or .gitignore + package.json accordingly; extraFilesCommitted lands in the result for tests.
• The push snippet was inverted — gated on !firstRun, i.e. shown only to users who already had repos and hidden from the brand-new users it exists for. Always rendered now with a firstRun-appropriate lead-in.
• "You're all set." above visibly unfinished steps: steps 4/5 have no completion detector; they now wear an Optional chip and the celebration claims exactly what was verified ("Core setup complete").
• CODEOWNERS removed from step 1's claims (bootstrap never writes one — only a parser exists); step 3's button label matches its destination; the foot help link points at /help instead of the API reference.

Typecheck clean; first-repo-scaffold + migration-onboarding suites 18 pass.